I found the cheapest way is to have a cheap Telstra Next G phone, in my case a ZTE. This now costs $15 per month out of contract including $15 of calls.
Originally AFAIR it cost $20 per month and zero up front, and included a wired in car kit.
When I want to travel I call Telstra (NB not Bigpond) and buy a data pack for $79 for 6gigs now, used to be less gigs. On the day I arrive home I cancel the pack. The packs are monthly and are prorated so for say 10 days you are charged half and get half the gigs. The only hassle is I use webmail to ensure friends and relatives are not sending bulk size emails.
I don't know ifthe economics have changed since then but I have found it great.
Regards Philip A
PS now it is 6 gigs I think that I could stop doing the webmail bit , but it used to be only 2 gigs AFAIR.

