Voluntary amendments can be filed any time before payment of the fee for the grant of the patent, except -

a) after a request for a search report (Patents Form 10) has been filed and before the receipt of that report by the applicant;

b) after a request for a search and examination report (Patents Form 11), examination report (Patents Form 12) or supplementary examination report (Patents Form 12A) has been filed, unless the amendment is contained in a response to written opinion (Patents Form 13A); and

c) after a request for examination review (Patents Form 12B) is filed.

An application for an amendment to the specification is to be made on Patents Form 13. The form can be viewed  here.

Note: Request for Supplementary Examination Report is applicable for application filed before 1 January 2020 only.