Yes, may I support Sue’s post. I have not noticed your NZ threads, so perhaps I am duplicating suggestions already offered on that Board.
But on the offchance that I am not duplicating other RChatters,
I wonder if you have the birth certs for any of your Grandma’s siblings? Particularly any of the birth registrations for those born in Victoria? You see, Victoria birth certificates are recognised as being perhaps the most informative certificates in the world.
The details on these are listed at the following RChat thread :
http://www.rootschat.com/forum/index.php/topic,373754.0.html So you should expect to find on those Vic certs,
confirming info or perhaps new info giving you
Mary’s names, including maiden name
Her occupation
Her then age,
Her birthplace (and from my own reading of the very few births in my own tree that were Vic registered) I can see that where my 19th century relatives were born ‘overseas’ that even in those instances, the birth place of the parents included the name of the village, the county, and then the country.
And of course all the other vital family history info you have mentioned in your Opening post.
So then with that info, perhaps we can try to find Mary's parents

As you have not yet provided the names of Grandma’s siblings, it is not possible to use any resources to look up the Vic BDM reference nos to help you locate those Vic birth certs.
But for around $20 Au, and your plastic card, you can immediately download the Vic BDM images, once you have found the year and registration no. There are quite a few RChatters who can assist in helping you with the reference no and year, once they know which names to look up.
https://online.justice.vic.gov.au/bdm/index-search?action=purchaseImage ADD
On the offchance, here’s several possible births registered in Victoria
Maria MINTON,
Father as Thomas MINTON, Mother as Mary MURPHY
Birth at Hotham in 1872
Registration no. 10606
And
Arabella Georgina MINTON,
Same parents named on the index
Birth at Collingwood in 1874
Registration no. 7912
Cheers, JM