Unfortunately the step from DIY to pressurized is expensive. Fortunately, it's 90+% an up-front expense, but, still, expect to spend $150-200USD (in the US - again, no idea about Aussie pricing and availability) for a complete prebuilt low-end rig and CO2 cylinder. A paintball cylinder, plus adapter, is only going to be $20-30 less than a 5lb cylinder, and needs to be refilled four times as often, with the benefit of saving a good bit of space. If you think you can swing that much cash, look into it, and many of us will be happy to answer specific questions (and there are many many threads about it as well). If not, I'd advise saving up if you are determined to make the leap, rather than going with a cartridge system. Even with *only* a 90L tank, the cartridge cost will be ridiculous within months.
There *is* an option of going with a paintball rig without a regulator, but there are definitely dangers associated with it. I'd advise a cartridge system over this method - and I *HATE* cartridge systems. It would cost you $50-75.