There are five main mobile carriers in Australia

Vodafone – www.vodafone.com.au 
Optus - http://www.optus.com.au/home/index.html
Three - http://www.three.com.au/
Virgin Mobile - http://www.virginmobile.com.au/
Telstra - http://www.telstra.com.au/mobile 

The most common carrier for Backpackers in Australia is Vodafone, so that is the one we will talk about.

It’s really easy!

Top up       You get        
$29     -        $150 
$49     -        $350 
$79     -        $550

For some reason in Australia when you spend $29 you get $150 of talk and text to use, it’s great!

Some advice, if you feel you might phone home to Ireland, get the $49 or $79 caps, as you get much more value for money.