Before you are put off by the process of gaining access to the records, be assured that lots of people are successful in doing this. Archives NZ holds the registers and the Poriura Hospital registers are at the Wellington branch.
To get permission to view the registers or to have someone in Wellington view it for you, write to The Ministry of Health, The Mental Health Section, PO Box 5013, Wellington.
Kenepuru Hospital records department hold the patient files and permission I'm told, needs to be obtained through the Official Information Act.
If you are like me and have missing people, especially if you have a birth certificate but no further information, this avenue would be good advice. Children were taken in as mental defectives and never heard from again. Their death certificates will turn up in the usual way or not maybe, I'm not sure.