Exetel - Wireless Broadband massive 2GB for $22.50 per month, no contract.
Plans with larger download limits but equivalent value are also available, as are $5 plans with no limits and 2.5c per MB.
As a comparison, Tel$tra charges $30 for a mere 200MB a month - only one tenth of this deal. Also, additional MB are only 5c each and not the 25c youd pay elsewhere.
Although I haven't taken it up, it seems a perfectly legitimate offering that started just recently on 1 March 2009. It's a HSPA service via the Optus wholesale 3G/HSPA mobile network. Exetel provides a USIM and/or USB STICK to provide you access to this.
Amazingly you also get a VoIP account (telephone calls via the internet) with a DID (Direct in dial - normal telephone number for people to call you). The VOIP offers really cheap calls to anywhere in the world, including 10 cent untimed calls to Australian landline numbers, whether they're next door or interstate.
To further add to its attractiveness, there seems to be a whole bunch of other bonuses including 30 free SMS, and 200MB hosted web space.
You'd have to do some research but you could potentially ditch your landline and ADSL internet in favour of this cheaper deal, which would save about $60 a month ($700 a year!), but likely more.
Follow the link to see their website for details. It would be interesting to hear from anyone who subscribes to it.
I have no affiliation with Exetel, but have been very pleased with their Naked DSL package for about a year now, which you can also see on their website.