After they've been turned, the full moon will cause unexpected phenomenal effects on them, which is different every month due to planetary alignments.
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.