The official announcement on the producer's instagram.
It's official! Cariba Heine is the H2O: Just Add Water girl to guest star on Mako Mermaids. Here is the official tweet from Johnathan M. Shiff the producer of H2O and Mako Mermaids.

"Welcome home to Mako...RIKKI! A very special guest star #makomermaids #H2OJustAddWater

You will see some screenshots from his INSTAGRAM. As well as Cariba's first public instagram screenshot.

I"m so excited! Are YOU?
Cariba's first official Instagram that's public.
Announcement with the mermaids tagged.