If you are in business (or renting out commercial properties) and your annual revenue exceeds $75,000, you must register for GST. The threshold is not based on a financial year. It is your turnover for the last 12 months. Be careful, if you go over the threshold and fail to register for GST, you can be liable for GST on income despite the fact that you have not charged your customer GST.
You have the option of registering if your annual revenue is below the threshold. Depending on your industry, registering when you are under the $75,000 threshold may be tax-effective.