Walter Richard McElwain married Charlotte CALVERT on 17 Jan 1866 in the the Presbyterian parish of Papakura. She was about 30 years old and he was about 39.Can I assume she was a widow of a man with the surname of Calvert, since you have her surname as Percy? Once her maiden name is cleared up I will see what I can find. Ordering the printout of their marriage certificate may provide additional information.Order at https://www.bdmhistoricalrecords.dia.govt.nz/Home/
The first record on Walter Richard McElwain in New Zealand that I found was the 1853-1864 Electoral Roll, which has him living in Auckland, and working as a leaseholder farmer.Also in Auckland are John McElwain (farmer) and George McElwain (gaoler).
Walter (of Waiuku), Charlotte and their 2-year old daughter Kathleen Mary are all buried together in lots 5-C-136 & 5-C-138 in the Wesleyan section of Symonds Steet Cemetery. Walter died 28 Dec 1900, Kathleen died 8 Dec 1878 and Charlotte died in 1908.
Walter's probate was filed 1 Feb 1901 at Auckland court.
Other children of Walter and Charlotte were Walter Richard Jr., born and died in 1871; Percy (1873-1930) and Richard (1875-1959).