Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It's expensive as hell, but Telstra is the best network in Australia (hands down).

I've tried all of the others; including Optus, Vodaphone and 3 - and none of them can even compare slightly to Telstra.
 
Telstra.

They're the most expensive but they have by far the best coverage and the fastest network with DC-HSPA and the largest LTE network too.
 
Hate to say it as a Vodafone customer, but if you NEED the speed, go with Telstra. You will pay through the nose for it though. Check the maps on their site for coverage and types of service of your area.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.