Get a Three mobile broadband prepay SIM and pay for whatever data you want. Then it will work in Australia, free. Job done. A prepay SIM will have roaming enabled (I'm 99% sure so might need to check) and a mobile broadband SIM is allowed in a tablet, as well as with tethering.
You can also get a regular Three PAYG SIM and pay for a £15 add on that gives you 300 minutes, some texts and unlimited data for 30 days. But these won't (technically) work in a tablet and you cannot tether. It will be detected and stop working.
Best of all, you can roam on any network Three has an agreement with, so it probably means better coverage.
Vodafone has today launched WorldTraveller that allows you to use your UK minutes, texts and data in Australia, but it's £5 per day! Not terrible, but not great.