Metamorphosis is a magical transformation that happens when a human jumps into a Moon Pool when the Full Moon is up and becomes a merpeople. During this time, any human that stays in the Moon Pool will be transformed into a mermaid or merman and given special powers over water.
The same thing occurred to Gracie, Louise and Julia in the 1950s in Mako Island. Decades later, Cleo, Emma, Rikki and later Charlotte became mermaids in the same place. Evie McLaren also became a mermaid there.