Hi Chris. He could have been. An NZ marriage search shows 27 results between 1882 and 1923 with groom John Millar (some with a middle name as well as John).
Here is a source reference for the marriage to Christina Jones.
<ref>New Zealand Te Tari Taiwhenua / Department of Internal Affairs [https://www.bdmhistoricalrecords.dia.govt.nz/ Births, Deaths and Marriages Online]. Marriage registration number: 1923/2059. Bride's given name: Christina. Bride's family name: Jones. Groom's given name: John. Groom's family name: Millar. [https://www.dia.govt.nz/Copyright CC BY 4.0]. Accessed 3 February 2023.</ref>
Here is a birth record:
<ref>New Zealand Te Tari Taiwhenua / Department of Internal Affairs [https://www.bdmhistoricalrecords.dia.govt.nz/ Births, Deaths and Marriages Online]. Birth registration number: 1867/13449. Family name: Millar. Given name: John. Mother's given name: Isabella. Father's given name: Robert. [https://www.dia.govt.nz/Copyright CC BY 4.0]. Accessed 3 February 2023.</ref>
And a death record:
<ref>New Zealand Te Tari Taiwhenua / Department of Internal Affairs [https://www.bdmhistoricalrecords.dia.govt.nz/ Births, Deaths and Marriages Online]. Death registration number: 1955/20337. Family name: Millar. Given name: John. Age at death: 88Y. [https://www.dia.govt.nz/Copyright CC BY 4.0]. Accessed 3 February 2023.</ref>
I'll add those to the profile if you like.