You would be surprised how good the rates from compoanies like avis, thrifty and hertz are. I'd suggest you check out http://www.thrifty.com.au

Or try http://www.budget.com.au

Most of the larger rental companies include insurance with an excess of $1,000 or something which you can lower if you want.

There may be a surcharge picking it up from the airport, so u could take a train directly in the airport terminal to kings cross and pick it up from that location.