Hi Neil,
Yes, source for the marriage/childrens births was the NZ BMD which is in part available online here:
https://www.bdmhistoricalrecords.dia.govt.nz/Home/(use the Search option on the left hand menu).
It appears as you say that the additional 2 children died in infancy (same source):
Deaths
1868/9299 Massey Alfred James 3M(onths)
1873/32 Massey Ada 10W(eeks)
Derek