Is the story of Mako before or after H2O?
Differences in time
Mako Mermaids shows a whole community of Mermaids called "The Pod" where mermaids aren't a very rare thing. H2O however shows mermaids that were always born human and live human lives. So does Mako take place years before and full mermaids went extinct?
In Mako Mermaids, the characters are seen owning high tech devices such as smartphones, unlike H2O. This may mean that it did happen afterward. This may not, howerver, be the case because that and laptops are one of the only differences.