Best Sim for Data (short term) ?

Hey guys,
I am looking to purchase the best value for money sim to use data with, for a week away (not long term).
Something with 20gb+ would be ideal. It will be going in an iPad, so data only is fine.
Don't care too much about the network, either Optus or Telstra should be ok.
Ideally a deal where you sign up and get the first month cheap would work well..
I am sure others out there have done similarly and I would very much appreciate what they have done to achieve this?