May I suggest you send an email to the NSW BDM general enquiries about the notation.
I think you may find that Brenda at some time after her 1930 marriage found that her nee surname was not Vernon but Hanson and perhaps was concerned if the information she had provided in 1930, and now known to be incorrect, would be causing her marriage to be voided.
I have a NSW mc post WWII where both the bride and the groom discovered that the information they provided at the time of the marriage was incorrect, (wrong fathers, completely different surnames, different dates/places of birth etc) and they both went to the NSW BDM with their respective mothers and sorted it all out. Their marriage was not voided, but the entry in the NSW BDM Reg Gen's records was amended.
The earlier information was struck through with a horizontal line (allowing the earlier text to be legible) and the amended information was placed at the side margin and through as much "blank white space" as possible. It has FIVE NSW BDM Initials, etc. One of the witnesses and the bride are still living, otherwise I would give more details.
There can be other reasons for notations on m.c. Hope this helps you
General Enquiries
http://www.bdm.nsw.gov.au/contactUs.htmCheers, JM