Best ask the D&G forums. You'll need an Oz CPL for starters. I think "Gwydir Air" is a good place to start looking for info.
Col Paye at Scone, NSW might be worth a look too.
From what I have gathered, you spend a fair bit of time on the ground being a groundy initially. Get a feel for the chemicals and the job that way. Tailwheel time is useful. You have to get an ag rating and work your way up.
Have a look at the CASA website for more info on the regulatory side of things.