Hi Patsy
Happy to have been able to help you out with this. (Always love these extra challenging ones).

Mmm ... have now just found that son William and his wife Ellie were also in Christchurch for a time.

Sadly there is a death of their small child, Marshall John, in 1896 (buried also at Linwood) ... and there may have been other children born to them in ChCh ?? ... so it's possibly worth hunting through the Library Index cards.
NZ Birth1894 - MASON - Marshall John Parents: Ellie Gertrude and William MASON
[NZ BMD online has other births from 1884 which could be for same couple ... but with spelling of mother's name given as "Ellen" / "Helen" and ~ maybe "Ethel Gertrude" (child Ethel Maud born 1885) ?? ]
The son "Albert William" (1889 - parents: Ellie & William) I found living in Gisborne with his mother in 1914. He went on to marry and have a child, but his life ended tragically.
William MASON though is a bit of a mystery still ? [Last on e/roll at Chch in 1911 ... wife and son in Gisborne from 1914, ... and wife still showing up as "married" ?? ]
~ Lu