Go for it.
The money you've been offered is great for Oz, and with exchange rates at the moment and forecast for the future you could easily send a good deal of money back to the UK if you wanted to.
Shopping costs are very comparable. Yes you'll pay more for some stuff but others will be cheaper. Location wise you'll be out in the sticks, and even Perth counts the same in many respects, so you'll possibly pay more because of that too, but even then you'll be way ahead because of much lower housing costs etc. Telecoms are the biggest rort (rip-off) you'll probably notice, but fuel costs more than make up for that.
I could write reams but don't have internet at home at the moment, only when I'm on the train or visiting friends etc. If you've got any specific questions then I'll be glad to answer, just send a PM.