You're correct on the second point. Technically it cannot be 'evidence' because I can't 100% prove it. I draw what I consider an accurate conclusion by process of elimination and say it's 90% Polly Tyree
Based on the age of the girl in the portrait being somewhere between age 14-21.
https://collection.nelsonmuseum.co.nz/objects/37684/tyree-miss-mhttps://collection.nelsonmuseum.co.nz/objects/56649/tyree-missThere are two other close family members taken in one of the sessions with her around the same period.
Firstly Mary Ann Cross Tyree, who was Polly's sister-in-law/wife of the photographer and Polly's brother William Tyree Jr., b 1848.
https://collection.nelsonmuseum.co.nz/objects/41157/tyreeSecondly William Jr Tyree and Mary Ann Cross Tyree's son James Arthur Vivian Tyree b 1878.
https://collection.nelsonmuseum.co.nz/objects/35349/tyree-mrs-and-arthurI can compare the appearance of age between them and to other studio portraits of the era to get the general idea this session was likely done between 1885-1889. I then can compare them with the portraits of 'Miss/M Tyree' to get a pretty good idea of her age.
'Miss M. Tyree' is the title as noted in provenance by Nelson Provincial Museum upon accession of the studio collection. I assume this to be Mary Hodgman (Polly) Tyree, as there were 11 siblings in total from William Tyree Sr.'s various relationships and she was the only one whose name began or had middle name M. Now looking in that local tree branch and surrounds for any females who would fit the age bracket; there are maybe 3-4 in this local branch and close surrounds and she is the only M. William Tyree Jr. and Fred Tyree only photographed family members pretty immediate to their branch of the tree at Tyree Bros Studio. Sisters, brothers, in-laws, their children, nieces, nephews.
Why am I convinced it’s her? Because this same girl did sessions with 'Miss/Kate Tyree'
https://collection.nelsonmuseum.co.nz/objects/37124/tyree-misshttps://collection.nelsonmuseum.co.nz/objects/37901/tyree-kateWhile 'Miss/M Tyree' looked fairly similar to her sister Kitty Cannon (Katherine Tyree), b 1871 they were not identical, they had different noses, and Kate was to be frank a bit prettier. With a number of images taken at the same time , it is fairly obvious they are not the same person and you can ascertain the difference.
At the end of this, whether it's proven 'Miss/M Tyree' in the portraits is Polly or not, is a moot point because all it would do is underline she was still around family until about 1890 before she 'disappears.' This would not as it stands actually answer any questions.
I can't recall where the information came from on her nickname being 'Polly.' This is nearly ten years ago now. The same source that quoted her sister Katherine Tyree as 'Kitty' and 'Kate' to the family. Probably a tree by a descendant on Mundia, now defunct and non-existent as reference. 'Polly' was a common nickname given to Mary and more so Mary Ann, for no explicable reason in the same way Keiths are often nicknamed 'Nobby.' So this is not unusual or out of the question and again, doesn't really have any bearing on answers to the mystery except as an additional tool for search terms.
Hello
Was also just about to query why the title of this topic has her name as
"Mary Ann (Polly) TYREE" ?? 
Incidentally, her mother's christian name was spelled as "Ann" (Ann Catherine) in other records (death notice / death 1885 etc. )
Also curious to know how it is that you have knowledge that her "nickname" was "Polly" ??
You have said ...
"Two pieces of evidence that Polly actually existed
... NZ Birth record listing 1872/697 [ yes
)
but ... the photo from Nelson Provincial Museum - ex TYREE Bros. portraits ... cannot really be regarded as "evidence of existence".
~ Lu