To be able to register for the Goods and Services Tax (GST) you must first have an ABN. You can register for GST through the Business Entry Point or the ATO website.
You MUST register for GST if the following applies to you: your annual turnover over the last 12 months has exceeded $75,000 and your annual turnover for the next 12 months is expected to exceed $75,000 pa. (There is a different level for not for profit organisations.)
If these conditions don’t apply to you, you do not have to register for GST. The advantage of registering is that you can claim back, on a regular basis, the GST you have paid on goods and services bought for your business. The downside may be that this involves you in extra accounting and reporting work. Every business needs to investigate these issues and decide for itself.