• NHI's uses the letters A-Z without I or O.
• NHI's start with three letters and end with four numbers.
• The last digit is a checksum.
• NHI's are assigned randomly. I believe hospitals have distinct sets ready to be assigned.
• Its possible for somebody to have multiple NHIs (first alphabetically is the primary).

The current Id. structure only allows for around 13.8 million NHI's. With a population of around 5 million in 2019 and the possibility of a single person being associated to multiple Id.s, I suspect NHIs in their current form wont be around very long.

References

(last checked 2023-06-06)

1. https://en.wikipedia.org/wiki/NHI_Number
2. https://www.health.govt.nz/our-work/health-identity/national-health-index
3. http://www.midcentraldhb.govt.nz/PatientsandVisitors/GeneralInformation/Pages/NHI.aspx

[EDIT]: Thanks to Chris W for pointing out a bug in my previous implementation. If the checksum is a '0' before being subtracted from 11, the NHI number is deemed to be invalid. This is now fixed in the generator.