Apart from a NSW BDM death cert, the other most obvious place to read of a NSW marriage is on a NSW BDM birth certificate... So, did your elusive Great Aunt give birth in NSW? If so, do you have an official transcription of that birth certificate? The information on the d.c., provided by a staff member at the San is likely drawn from info gleaned that staff member's recollections of conversations with the the patient.
You note that her death is registered in the name of Elizabeth Margaret MATTHEWS, in 1949. (#11079). I have checked the online index from NSW BDM and I note that Elizabeth Margaret MATTHEWS is how it is indexed, and it is indexed only the ONCE.
To me, that is indicating that she was known by the surname MATTHEWS .... which you have as her maiden surname. How certain are you that the lass who died at the San is your lass? A search for the formal ending of a marriage can be exhaustive, afterall it may have ended interstate or overseas....
Have you sought the d.c. for her husband as per the d.c. info ... when/where does it have him marrying your lass ... and does it have any children of their marriage? I cannot see any born to the couple via NSW BDM online index...
Here is the guide for NSW marital matters prior to the 1976 changes moving these matters to Federal jurisdiction...
https://www.records.nsw.gov.au/archives/collections-and-research/guides-and-indexes/divorce-records-guidehttps://www.records.nsw.gov.au/search_form?id=16Here is the online index for those NSW matters
I have requoted your Opening Post, and spaced it to separate each of the facts you have found.
I have spent many years tracking down an elusive great aunt, who may actually be my g/grandmother.
Finally found her buried at Waterfall Sanatorium Cemetery (for TB).
She was born Margaret Elizabeth MATTHEWS in Sofala, 1884 to Henry and Annie.
I've just received her death certificate which lists her as Elizabeth Margaret MATTHEWS died age 64 in 1949 at Sanatorium & is buried there. (nb there's no public access to this disused site.)
It states she was married to William MORRIS at the age of 19 in Balmain, NSW (so about 1903).
Cemetery records say she was also known as MAY MORRIS.
It lists her address as 21 Shellcore Rd Chullora NSW but it doesn't seem to exist & think it may be Shellcote Rd Chullora?
I can find a William Morris living at no 10 with a Lillian May Morris, but Lillian is still listed on electoral roll at that address in 1963 so that can't be her.
I can't find any marriage record on NSW BDM nor anything on ancestry.
Can anyone give me an idea where else I can locate info on her marriage to William Morris etc. (forgive long post - family rumour is that she was ostracised for illegitimate child, which may actually be my grandfather, Walter. He's listed as child of Henry & Annie too, ie her youngest brother (born 1907) but it was common back then to “pretend” you are the parent when in fact you are the grandparent if your daughter had child out of wedlock).
Have you checked the birth cert for Walter to confirm that his older siblings are listed there, and that your great aunt is listed there with her then age as at the time Walter was born?
Have you checked the d.c. for both Henry and Annie to confirm that he is listed there as their offspring?
Are you certain that 'there were no children of the marriage' of William MORRIS and your great aunt? NSW ER 1902 DALLEY,
polling Annandale
William Prosser MORRIS, 195 Parramatta Road, Bootmaker
polling Leichhardt
William MORRIS Paling St, labourer
NSW BDM 'Balmain District' 1900s - far larger than just the suburb 'Balmain' ...
JM