there is a place in north melbourne that fit and sell car seats at competivive prices. the best thing about them is that you take your car there and they will tell you which car seats fit better, and physically show you different combinations in your car. I do believe that you don't have to buy off them if you don't want to. its a no obligation service, but they are pretty competitive. They also will fit them for you. I think they advertise in Melbourne Child, or otherwise try the yellow pages.
HTH
also most car seats in australia are made by the same company, but they different suppliers put their own brand on them. They all have to meet specific standards, and the difference between a cheaper end one and a dear one is the material on the cover and any gizmos or gadgets that go with it.
the plastic mouldings etc, are all the same. its just the look taht is different. The width does vary with several models, but the width of a car seat does not impeed on its safety standards.
Good luck hunting