I just came back from perth.
Some useful information/tips
I got 2 pre paid cards from "yes optus" and brought my own phones
the SIM card is free and i put A$10 credit in each, so it lasts me 5 days.
if you need to make calls back to spore it costs A$1 for 10 mins, just add credit if needed
i used their "$2 days plan"
so for A$2 per day, i get unlimited local calls in australia, unlimited SMS and unlimited data.
I use the unlimited data to make IDD calls and use the map and search stuff.