To give an idea to those who are about to arrive.
there are booths in the airport like voda and other mobile carriers. you can get your prepaid sim there. They will register your new number with your passport details.
You can later on switch to cheaper mobile resellers like TPG, Vaya.
You can retain same number since they support porting. I am using postpaid Vaya that piggybacks with Optus its a $17 plan with 1.5gb data and $650 worth of calls and texts (doesnt support international calls)