Sunnyside Hospital, Christchurch’s first mental asylum, was opened in 1863. It was built to house those who were considered insane, until then held at Lyttelton jail.Archives New Zealand in Christchurch holds registers of admission, death registers, casebooks from 1854-1956. Patients sometimes had a page of information to view in the casebooks.
The patients have been indexed to around the 1920's at the archives, but there is a 100yr restriction on viewing these records so even if your person has been indexed, you may have to wait awhile or write to Canterbury District Health Board's solicitor for permission outside of the restriction.
The patient's names will not be listed on Archway, but you can search for what National Archives holds on Archway.