I made an error in my GST return. What should I do?

From IRAS web Site:

If you have made a mistake in your past GST return(s), you can correct the error in the current GST F5 return provided that:
 
  1. The net GST amount in error for all the prescribed accounting periods affected is not more than $1,500; and 
  2. The summation of errors in each prescribed accounting period is not more than 5% of the total supplies. In the case where there are no taxable supplies in the accounting period, the 5% rule shall be applied in respect of total taxable purchases.
If any of the above conditions is not satisfied, please correct the error by submitting a “Disclosure of Errors on GST Return” i.e. GST F7 for the affected prescribed accounting period(s).