GST is optional if you gross under $75,000. If you decide to register for GST (for example, if suppliers request it - I know that Synnex, an IT distributor, do), the ATO gives you an extra benefit of letting you process your GST yearly, at tax time, rather than 3-monthly. I believe this sticks until you gross $20 million - then you have to go onto the 3-month "Business Activity Statements" method.