Interesting story I found on PC Authority today regarding the best prepaid mobile broadband deals. It concluded the following:
We believe that 3 offers the best plans, at least when it comes to cost per gigabyte. It costs less than the others and it has fewer traps. Vodafone might be our second choice, especially since it offers better coverage than 3.
If you are looking for strong network coverage and consistant performance, then Telstra is your best best.
Obviously the larger the download quota, the better value the deal would be per MB/GB. You can compare all prepaid mobile broadband plans here.