Answer:
Mako Island is a fictitious place, filmed at Sea World on Queensland's Gold Coast. The scenes featuring the school are filmed at Somerset College.
For the purposes of the H2O television series, you can only go there whan it is a full moon. It's a fantasy show.
Sea World is located specifically on "the Spit" at Main Beach, Surfer's Paradise. If you wish to get to where the Mako island set was done, follow the signs from the Gold Coast Highway at Surfer's Paradise.
More information is available at the links below.