Alastair, you could try apply for a transcription rather than an actual certificate. Transcriptions show all information on the original certificate including details that are no longer included such as who was present at the birth,  and any changes made after the original registration [names etc].   I have obtained at least two transcriptions for registrations that have had the surname amended after registration. One change was through the Legitimation Act and the other was a second registration by the fostering father [child was born illegitimate] who claimed the child had not been registered at the time of the birth.   Also if the original birth was registered in a different surname, then that original registration should still be in the BDM  index under the original name.   For example, my grandfather was originally registered under his mothers maiden name in 1893. He had a second birth certificate issued in 1919 under the Legitimation Act in his 'supposed' father's surname. BOTH registrations appear in the NSW BDM Index with different numbers and dates.   Transcriptions can be obtained from a transcription agent. They are listed on the NSW BDM website.  I always obtain transcriptions rather than certificates as they are cheaper and will contain more information.    Feel free to contact me if you don't understand any of the above.   Hope this helps,   Jenny