Basically, if your business generates a GST turnover (gross income from all businesses minus GST) of $75,000 per year or over, you’re required to register and collect GST. If your turnover is below this threshold, you can choose whether to register.
If you’re registered, GST is typically included in the price of goods and services the business provides. You can generally claim a credit for any GST included in the price you pay for products and services your business buys or acquires.
This is called a GST input tax credit
For businesses with turnover of less than $20 million, the minimum requirement is to lodge a quarterly Business Activity Statement showing the net GST payable (or refundable).
You can find out more about GST and your requirements at the ATO website http://www.ato.gov.au/ or speak with your accountant or business adviser.