Telstra have just reduce the cost and increased the data allowances on their Post-Paid Data Packs. The good thing about these data plans is that you can purchase a new data plan at any time during your billing cycle to avoid having to pay any excess usage fees. As you can see below based on the cents per MB costing, the two sweet spots are either the 500mb or 3gb plans.
If you already have a data pack on your existing Telstra plan then you will need to call them to update your phone plan to take advantage of the new rates.
Form more information see http://www.telstra.com.au/mobile/browsing_packs.html