You can register for GST when you first register your business or at any later time.
If you’ve just started a new business and expect it to reach the GST turnover threshold or more in its first year of operation, you should register for GST.
If you’re not registered for GST, check each month to see whether you’ve reached the threshold, or are likely to exceed it. If your turnover exceeds the relevant threshold, you must register within 21 days of reaching it.
You only register once for GST, even if you operate more than one business.
If you do not register for GST and you are required to do so, you may have to pay GST on the sales you have made since the date you became required to register – even if you did not include GST in the price of those sales. You may also have to pay penalties and interest.