do NOT over look insulation. Where abouts in aus are you? It can be pretty sunny, might think about incorporating those black tubes on your roof like they have for pools, along with either a heat exchange with a water hater, or a bunch of electric heating.
Lets get this straight, you have a 17000L tank and the sky light is 3 levels above the tank!
I am assuming that this is not in a house?
Any pics of your tank?