I'm sure you could find marinas if you google search but truthfully most people would trailer a boat of that size. The whole of Aus is catered very well for trailer boats with good ramps and wash down facilities at most ramps. If it's fishing you are after there's all sorts on offer from very good snapper fishing to King and samsone fish on offshore reefs. Towing the boat to places like Port Lincoln is a top place to hunt down giant southern bluefin or even jump in with them.
Diving is a little tougher as the water is cooler down that way, as well as more great whites in the cooler water.
Over here most people don't think twice about towing their boats long distances, I've just returned from a trip north where I towed my boat over 6000 km for 4 days of spear fishing.