HI Leon
Vodafone do a wireless broadband plan at $39.95 per month for 5Gb downloads. That's heaps unless you want to download movies etc. They have pretty good coverage on 3G network in capital cities and GPRS ( sort of wireless dialup speed wise ) in expanding country areas. You can check out their coverage maps to see the extent of their network as applies to where you may travel. I'm with them here in Perth .... right now in fact ..... I tend to agree that Telstra seem to have the widest coverage but things are changing rapidly so do your research.
Good luck mate
p.s. If you sign up with them for 24 months you get free USB mode ( worth $300 ) that fits in your pocket.
p.p.s. Optus are doing similar plan now as well.
As a footnote .... internet access is available just about everywhere now in caravan parks, hotels, motels etc etc etc.