For coverage almost everywhere you cant beat Telstra Bigpong, but you pay accordingly.
I have an
Exetel plan, uses Optus network, $5 per month, no quota and you pay for what you use at 1.5c per MB on top. Works fine in all major centres. In some places coverage is not 3G (and speed becomes like dial up), but I'm not willing to pay Bigpong prices to get better coverage.
The problem I have with prepaid is they expire and I may not use the bought quota.
Exetel works for me.
(Virgin mentioned by previous poster also uses Optus network).