Here's what I found from Wikipedia:
"In 2003 Parliament enacted the Alteration of Sex Description and Sex Status Act, which allows a transgender person who has undergone medical or surgical gender reassignment to apply to the Department of Home Affairs to have the sex description altered on their birth record. Once the birth record is altered they can be issued with a new birth certificate and identity document, and are considered "for all purposes" to be of the new sex.
The specific definition of gender reassignment in this Act refers to reassigning a person's sex by changing physiological or other sexual characteristics, and includes any part of such a process. Thus the transgender person is not required to have had genital surgery in order to have the sex description altered.
In 2013 it was reported, in response to a question in Parliament, that 95 people had legally changed their gender under the law. It was also reported that problems have been encountered by applicants because officials are not consistent in their interpretation of the medical requirements."
So it seems that SRS/GCS isn't necessary, however the wording is ambiguous. The best thing to do would be to call the bureau that handles this and ask them directly.