Thanks everyone for all this information. Whoever these people were, they are most definitely my relatives. While filling in all sorts of details from here on my family tree, I also started a shadow family tree for my strongest unknown DNA match. Within minutes I arrived at Agnes Lovelock STRATTON and William MARTIN....
I followed through with the Genealogical Society SA and was directed to GSNT, and they have supplied me a pdf of all the information they have on the Strattons. The most interesting says that Margaret MARSHALL was born in New Zealand in 1850, but the birth is unregistered. There is no sourcing for this information, so I will write back and enquire further. However the other details are all sourced and accurate, so maybe it is correct.
This would explain why no one can find a proper Scottish reference for her, but it would make the fact she is meant to be the sister of Glasgow born John MARSHALL of Westport somewhat mystifying. I can't find any other John MARSHALL living in Westport in 1880, although there is one close by in Greytown, and there is a John MARSHALL buried in Westport in 1883, but I think he is a 10 month old baby.
Other family trees on the web insist on identifying Margaret as someone born in the 1830s in Stirling, Scotland, or they just go with the 1850 date without further identifying her birthplace. I think most follow the too common ancestry.com 'copy whatever you find' method of research, so the same wrong data perpetuates. I have ordered the Straton's marriage certificate, so maybe that will give some more revealing information. At this point even where she is born would be useful!
Did we achieve answering what I originally asked? I guess so, as I learnt that the death records were not worth pursuing, but along the way I have learnt many other things as well, and I have a few new leads as to Margaret's origins. If anyone has any other ideas I am always grateful to the wonderful people on this website.
Thanks everyone.