Austar and Foxtel use the same satellite so I think the same dish should work. I would probably go and see a satellite installer in Darwin (just search for satellite in Darwin on the Yellow pages site) And request a quote for the smallest recommended satellite dish aligned to Optus C1.
Try and have a look at somebody else's satellite dish that has Austar (not SelecTv as that uses one of the Intelsat satellites) and that will give you an indication on the size required. Because you are in the top end the dish might need to be larger as In Tassie we can get away with a 95cm dish but I think the further north you go it gets bigger but the local people will know.
Up here I had a look at satellite dish, the majority of them are austar dishes, pointing due east (either northeast, east or southeast) depending on the optus c1 position, and those are on the roof.
The bigger the dish, the more expensive it gets? Correct? I had a look in yellow pages and they are heaps of shops that sell satellite dishes here