Travelling from the UK to Aus for 3 weeks - advice on gettting a SIM?

Hello Australian chums - and of course, congratulations on your win today at Lords.

At the end of the month me and the family are travelling to the West coast for 3 weeks to see family. Mainly spending out time in Broome, but we've got a 4x4 some small (carefully prepared, cautious) trips into more rural areas.

Our cellular UK provider's rates for using phones is ruinously expensive, so I'm looking for advice on

1. Which cellular provider you would recommend to get a - presumably pre-paid SIM while we are out there. Main criteria are

2. Probably want about 4-8Gigs data, mainly calls within Australia with probably about 3 hours of calling back to the UK during this period on one phone.

3. Anyone know whether it is possible to get the SIMs before we travel in the UK? Or best to get them in Oz? and if the latter - best place to get them?

Sorry for the shopping list of questions. Any advice gratefully received.