The classes are usually run by physios, so calling local places, especially if they specialise in womens health would be a good start. Ours is run at the local public pool, in the hydrotherapy area, so again if you called any swimming pools around they might have classes on.

google aquamums - there are classes in st kilda and armadale, and another site i found said pre-post natal aqua classes in ashburton.

A few things come up in the results so might be worth looking at?