GST is a self-assessed tax. This means that businesses are required to continually assess the need to register for GST. Registering for GST is compulsory when the turnover of your business is more than S$1 million for the past 12 months or is expected to be more than S$1 million in the next 12 months.
Once a person is liable to register, he should inform the Comptroller of GST within 30 days from the day the liabilities arises. If the total turnover of your company is more than S$1 million in any past 12 months and you have not informed the Comptroller of your liability, you should do so immediately.
If the Turnover of the company for the past 12 months is less than S$1 million, no action is required from you.
The only exception is if you are certain that your combined turnover for the next 12 months will be not more than S$1 million.