Is it possible to get ADSL and not pay line rental, as the cheapest I can find is $20 a month to rent a line that's not being used for anything but data, which I'll be paying for anyway.
Yep, it's called Naked DSL and several providers offer it. In some cases the ISP will be able to order the new line for you, but you may well need to get Telstra to install the line first and connect it up (this has its' own complications - definitely ask your ISP for advice), then get the ISP to convert it to a "Naked" line.