I'm off to Aussie soon and I've found an Australian $4.90 plan with 40G data and unlimited calling to AU and NZ numbers.
The data lasts 30 days.

It's the same Kogan deal that they have started offering in NZ, but Aussie based and seems to be a regular on and off promo in Australia (eg 4 weeks off 4 months on).

Sign up here. 

They mail you a sim so you will either need an address in Aussie to receive it, or it looks like you can buy a sim from 'selected' 7-11 stores and united petrol stations.

Within an hour I got an email with an activation code so sent to you, pop in the sim when you get to Australia and set it up.
(I suspect you will have to remember deactivate auto billing etc but I can do that)

Someone on the ozbargain site states that you can have up to 6 sims associated with 1 credit card number.
So between me and my wife credit cards we should have no trouble with 3 sims I have ordered for our family.