Here are some of the Australian 3G data plans comming up for the ipad.

Telstra
Australian Macworld
$20 = 1GB Data
$30 = 3GB Data
$60 = 6GB Data
$80 = 9GB Data
$100 = 12GB of Data
All expire after 30 Days
Prepaid I believe.

Optus
iTWire - Optus one-ups Telstra in iPad stakes
$20 = 2GB Data
$30 = 3GB Data
$60 = 8GB Data
I read this is prepaid too but not 100% sure.

Vodaphone and 3 will have their own ipad plans with information released in late May. I could not find any hard links on this so we'll just have to wait and see.

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Or the easy anternative I just found is to click on all the carriers links from the Apple Australia ipad 3G page
http://www.apple.com/au/ipad/3g/