So, doing some quick research I've found a few things that may be useful.
Just to make sure everything is properly legal, make sure you get the air tank from a shop that's actually in Australia. High pressure air is no joke, and each country tends to have their own rules. www.paintballshop.com
is in that country, can't say if they're any good though, they were just the first result on a search, but at least anything they sell should have the right stamps on the tank.
Second, you did fill out the proper forms for owning a paintball gun in Australia, right?
Each state and territory tends to be a little different, so pay attention to that. And if you're in the Northern Territory or Tasmania you can't have one at all.
And the quick and dirty guide for air tanks.
3000psi systems. Doesn't hold many shots, all metal tanks are heavy, but cost considerably less.
4500psi systems. Holds 50% more shots compared to a 3000psi tank of the same size, often come in larger sizes for even more shots, uses fibre wrapped bottles that are stronger and considerably lighter than the all metal
tanks, but cost considerably more.
CO2 tanks are way less expensive and can hold quite a few shots worth of gas, but NOT all guns can use it, and overall shot consistency isn't as good as high pressure compressed air. Find out what he used before.