No banks in Australasia can offer fixed rate mortgages in NZ for more than 5 years for 2 reasons :
1. Lack of depth and liquidity in the wholesale fixed interest rate market.
2. No hedging mechanism using government risk free bond rates as government bonds are mostly up to 5 years duration.
So don't expect it anytime soon.
https://news.anz.com/new-zealand/pos...ages-explained