Surname, had three children (no names), from Muriel Leo’s (nee Oliver, 1917-1998) Family Tree.

First given names from Matthew Cox in an email dated 3 June 2018. Matthew is the grandson of Grace Elizabeth McKoy (b.1887).

Matthew writes in the 3 June 2018 email that, William and Grace had four children: Raymond Charles, Hilton, Hazel and Brian William.

To try to determine year of birth and death, a search on the official Death Records on the New Zealand BDM website in June 2018 shows two possibilities:
Died 1945 Reg No. 21579, Cox, William Charles, 72Y [Therefore born ca 1873]
Died 1965 Reg No. 40888, Cox, William Charles, 77Y [Therefore born ca 1888 in New Zealand]
A search on the Internet by Brian of nzolivers.com shows the latter is the correct one.

Year of birth (1887), occupation, full date of death, year of marriage, date and place of burial, headstone photo, from:
https://www.findagrave.com/memorial/137509067/william-charles-cox

Birth: 1887/11550
Marriage: 1909/4844
Death: 1965/40888
Surname: Cox
First name(s): William Charles
Date of death: Monday, 27 September 1965
Cemetery: Ruru Lawn
Date of burial: Wednesday, 29 September 1965
Block number: 38
Plot number: 487
Age: 77 years
Occupation: Retired Grocer
Place of birth:
Years in New Zealand: 77

Year and country of marriage from search on New Zealand BDM website in January 2019:
1909/4844. Bride: Grace Elizabeth McKoy. Groom: William Charles Cox

Full date of marriage of 9 December 1909 from further searching in January 2019 on BDM Search by incrementally changing the date range for the marriage.