After they've been turned, the Full Moon will cause unexpected phenomenal effects on them, which is different every month due to planetary alignments.
The moon is the source of the powers that Cleo Sertori, Emma Gilbert and Rikki Chadwick receive in the Moon Pool on Mako Island. The moon also affects each of them in different ways, at various moon cycles.
A full moon has very strange effects on the girls and these effects are explored over a number of episodes. It seems that not only does the full moon give them an opposite take on their original personalities, (Emma: outgoing, and disobedient; Cleo: amazing singer, self-centered; Rikki: vulnerable and self-guilt), but makes them quite romantic and seductive, seeing as Cleo, Emma and Rikki have all kissed their love interest for the first time while under the moon's influence.These effects only last for as long as the full moon is in the sky. During a full moon, the moon pool on Mako Island sometimes protects the girls from the moon's strange effects. The girls usually don't always remember what happened to them when they were under the influence of the full moon.
Emma is the first to fall under the effects of the full moon, which turns her into a wild and carefree mermaid; exactly the opposite of her normally organized self. In this state, she freely tells everyone exactly what she thinks of them, regardless of whether it would offend them or not. It also causes her to change into a mermaid and is unable to transform back. While under the effect of the full moon, Emma feels inexplicably drawn to water and to Mako Island. She develops a craving for fish as well as a craving for Byron. She becomes somewhat seductive and kisses Byron on several occasions. By the time the moon is completely gone, everything is once again normal and Emma remembers nothing.
While Cleo and Rikki looked at the moon, they did not fall under its magic. Presumably, this full moon only affected Emma specifically.
Cleo is the second to fall victim to the full moon. Her powers resemble myths about sirens attracting sailors with their song to a watery doom. Cleo uses her song to attract many boys. In this effect, Cleo is more self-centered compared to her normal, compassionate self. While under this spell Cleo does not change into a mermaid when wet.
Rikki is the third to experience the effects of the full moon, which causes her Hydro-Thermokinesis powers and abilities to lose control and overload in a minute of time. Instead of merely heating liquids, she begins to heat up the air and set fire to objects around her by touch and by sight (Advanced Pyrokinesis). During this time, Zane comes to Mako Island to try and find the "sea monster", and discovers Rikki sitting amongst a circle of flames. Rikki then kisses him, causing Zane to quickly overheat and pass out, becoming dehydrated and gets sunburnt. The girls realize that Rikki's mermaid power and ability is the most volatile and must be tightly controlled.
In the season one finale, "A Twist in the Tail", Miss Chatham reveals to the girls that they are able to lose their mermaid powers if they are in the moon pool of Mako Island during a lunar eclipse takes place. The girls, knowing that Dr. Denman and her assistants would not let them rest until they got what they wanted, decide to take this opportunity and fight off Dr. Denman's men just before jumping into the pool and letting the moon draw away their powers. This helps the girls trick Dr. Denman into believing that they are no longer mermaids. Lewis later reveals that Miss Chatham told him that their powers and abilities are only lost for twelve hours, not forever, and that Miss Chatham did not tell them so that they would truly believe they had lost their mermaid powers and abilities and being mermaids forever. It is unknown if the full moon makes mermaid powers stronger.
In the first episode of the second season "Control", all three girls are affected by a very special full moon, which causes them to swim to the moon pool on Mako Island. Because of a particularly rare planetary alignment which was taking place at the time, their powers and abilities are greatly strengthened and increased to even higher power levels, and they use their newfound powers and abilities to attack and blow away Lewis, who followed them to make sure everything was okay. Following their reversion to their normal state, the girls struggle to control the effects of these new powers and abilities: Cleo creates a windstorm while upsetting a glass of orange juice her younger sister is carrying, Emma manages to freeze the entire kitchen bench as well as the fridge and freezer with solid ice, and Rikki manages to set her laundry on fire while trying to speed up line drying on her clothes line. She repeats this later in the episode with Lewis' shorts.
The next full moon is on the same date as the camping trip for the Sertori family. Their choice of location could not have been any worse: Mako Island. To ensure that none of the girls fall under the spell of the full moon, Cleo brings Emma and Rikki along. Lewis joins the trip as well, bringing Charlotte at her insistence. Even though the girls sleep in a "moonlight-proof" tent, Cleo is affected once more when Kim opens the tent, while searching for Charlotte, who had gone into the forest with Lewis. When Lewis comes back and warns the other girls, the two girls decide that they have been afraid of the full moon for too long, and venture outside to rescue their friend despite the danger. Both are instantly struck by the moonlight, but somehow manage to channel the effects and find Cleo. Rather than exhibiting ridiculous or flamboyant behavior, they become single-minded and emotionally indifferent, as well as being somewhat vicious in their reaction to threats. Charlotte follows Cleo to the moon pool and is about to touch the magic mystical water, but Emma freezes it just before she does, preventing Charlotte from becoming a mermaid. This is one of the two times that the girls remember what they did, as Rikki and Emma later talk about how Emma managed to freeze the water just as Charlotte touched it.
By the following full moon, the girls have finally sealed off the house completely. Things are spoiled when Ash Dove pays a surprise visit to Emma, and Cleo catches a glimpse of the full moon, causing her to become giddy and childish in her behavior. While Emma has her "date" with Ash, Rikki keeps Cleo locked up in the bedroom because she has been "moonstruck". However, Cleo causes Rikki to see the moon as well by pulling down the cardboard covering the window and the second mermaid falls into a similar state of giddiness. Together they lure Emma into the bathroom, toss her in the bath and make her fall under the moon's spell as well. They plan to let Ash see Emma as a mermaid, and he is on the verge of entering the bathroom when Lewis, being called earlier by Emma, bursts in and stops him. Later on in the episode, Emma admits her feelings for Ash and the two kiss.
By the next moon, Charlotte has become aware of her grandmother's past and uses the moon pool to become a mermaid herself. The other girls remain in Cleo's room and manage to avoid seeing the moon. When Charlotte becomes a mermaid there is a normal full moon but the strange thing is that not only does she have all three original powers and abilities (which is probably because the moon pool divides the three powers amongst those in it, so if there is only one person in it, she will acquire the three powers) but for some unknown reason she develops the additional powers and abilities as well.During the season two finale, "Finale", Max reveals to Lewis that every fifty years an extremely special full moon occurs, during which all the planets are aligned with the moon and earth, making its effect extremely strong and powerful enough to take away a mermaid's powers and abilities forever if they enter the moon pool while it is in its active state. However, this full moon also greatly strengthens and amplifies a mermaid's powers and abilities and makes them much stronger and more powerful, as shown during this episode when Charlotte suddenly develops a fit of anger, and uses her powers and abilities to make the plumbing and water system of Emma's house go haywire, out of spite toward the girls. Because Ash was in their presence, the girls were unable to fight back. Leading Lewis to believe that she was under the influence of the full moon, Charlotte lures him to Mako Island in order to trick the girls into coming after them. At the end of the episode, while the moon is over the moon pool, a big showdown begins at Mako Island. Charlotte tries to push all three girls into the moon pool while it is active, in an attempt to make them lose their powers and abilities forever, so that she will be the only "true mermaid." However, they then combine their powers and abilities to fight back and win, making Charlotte fly into the air and drop into the moon pool, resulting in her losing her powers forever. During the battle, Charlotte reveals that this full moon event is how Gracie, her grandmother, and one of the original three mermaids in the 1950s, gave up her powers and abilities fifty years ago.
In the third season the girls are now immune the affects of the full moon, however instead it causes an incredibly strong and powerful tentacle-like creature made of pure water from the moon pool and a strange new waterfall attack the girls, dragging them into the ocean and to the moon pool.
The water tentacle emerges for the first time in The Awakening. It captures Rikki and brings her to the Moon Pool. Just as the full moon is passing over, Bella and Cleo combine their powers and destroy the water tentacle.
In episode "Kidnapped" the creature captures Bella and attempts to turn into water. Cleo uses her power to keep Bella's body together until the moon passes over. Afterwards, Bella says while she was turning to water, she felt safe and at home.
Rikki goes to confront the water tentacle after a heated argument with Zane. She is the first to confront the creature and connect with it. While it tries to communicate with her, Will walks in on them and interrupts them. The creature attempts to attack Will, but Rikki stops it before it can touch him. After the moon passed, the water tentacle disappears.
The mermaids find out that the water tentacle is a friend. They confront it together, and it shows them a hologram of a comet moving toward Earth, but before the hologram can show everything, Zane and Will walk in and interrupt it.
The Comet Eva is about to fall toward Earth and collide with the Moon Pool when Sophie and Ryan plant explosions on the cave and subsequently destroy the magic within, as well as the mermaids' best shot at stopping the comet.
Knowing the comet will destroy the entire world when it collides with the Moon Pool, the mermaids create The Tower of Light to alter its course using their own powers.
On the first full moon after his transformation, Zac feels strongly drawn to the moon's pull as it rises and changes into a different personality when he gets hit by moonlight, and goes toward Mako Island. After the full moon, Zac returns to his regular self but develops the power of invisibility.
On the second full moon, the same thing happens again and he goes to Mako to claim the trident. Unlike the first time, he remembers everything that happened, including the mermaids' true identities. It is unknown whether or not Zac has developed any new powers.
Special full moons
On special nights the full moon is different and affects the girls in different ways. On the night of a planetary alignment, if any mermaid is in the Moon Pool when the full moon passed over head there powers will become stronger, causing the girls to not only have water but also weather powers. This becomes even stronger when all of the girls put their powers together, they then have complete control over the weather. This can cause electrical storms. On other special full moons such as a Lunar Eclipse if any mermaid is in the Moon Pool when the moon falls over head shall have their powers drawn away for 12 hours, perfect for if you have someone who is trying to expose you, such as Dr. Denman is "A Twist In The Tail". When all of the planets align with the moon, which if any mermaid is in the Moon Pool when the moon comes over head they will lose their powers forever, which is what happened to Charlotte Watsford in "The Finale".