I'm sure you've already found this, but John Gray A. is registered on NZBDM as 'Arnett' in 1863. His parents aren't listed there. A relative also researching the family tree has copies of the birth and death certificates, listing John's parents as William Arnett and Hannah.
John Gray A. unfortunately died of a mental health disorder in the asylum at Hokitika in 1899. He's buried in the Arnott Family plot in Karoro Cemetery with his parents. As I'm sure you can imagine, given the times, he wasn't often spoken of nor were many records kept of his life. In fact, some of his good deeds (including rescuing a woman and her children from a foundered ship) were attributed to others!