If you haven't registered for GST, and you become aware that your GST turnover will exceed the $75,000 per year threshold, you will have to register for GST within 21 days.
It's a good idea to check each month to ensure you're not likely to go over the over the limit. Keeping an eye on your GST turnover is important so you can register if necessary. If your GST turnover is below the $75,000, registering for GST is optional.
You may choose to register if your GST turnover is below the $75,000 threshold, however this means that once registered, regardless of your turnover, you must include GST in your fees and claim GST credits for your business purchases.