The National Health Index (NHI) number is the unique person identifier used within the New Zealand health system. It is technically not a number but rather an alphanumeric identifier consisting of 7 characters, with three letters and four numbers. It is often referred to as the NHI, although care must be taken when using this abbreviated term, because the NHI can also refer to the national collection of health care user demographic data (of which the NHI Number is the unique identifier).
The NHI Number, as part of the NHI was established in 1993.
Primarily the NHI is used to identify individuals uniquely within the New Zealand health system, especially in electronic systems. An example of this is its use to alert health care providers using the Medical Warnings System (MWS) of risks associated with medical decision-making for specific patients.
An NHI number has a specific format. It is 7 characters in length and contains a check digit. This format consists of 3 alphabetic characters being in the range of A-Z, excluding I and O, and 4 numeric characters being in the range 0-9. The 4th numeric character is the check digit. The assignment of the first 6 characters is arbitrary and bears no relationship to the individual to whom it is assigned.
The NHI Number is most often represented with the alphabetic characters upper case.
NHI Numbers are often referred to as being valid or invalid. Any NHI Number that does not fit the correct format or that has an incorrect check digit is referred to as invalid. Usually reference to an NHI Number being valid or not does not indicate that it is correctly associated with the right individual. As the identifier is arbitrary there is no way to do this based solely on the identifier itself.
When it has been identified that an individual has been assigned more than one NHI Number, one is deemed to be the primary identifier. This is usually done by ranking all assigned numbers in alpha-numeric order and choosing the first one as the primary.
All other NHI Numbers for the individual within the NHI are then linked to the primary one.
Check Digit 
The NHI Number contains a check digit. The algorithm for generating the digit is described below:
Each alpha character is given a numeric representation equivalent to its ordinal position within the alphabet, starting at A through to Z. The letters I and O are omitted making the ordinal range 1 - 24.
Each alpha character's numeric representation is multiplied by the inverse of its ordinal position within the NHI Number. The first value is multiplied by 7, the second by 6 and so on.
The first 3 numeric characters are multiplied by the inverse of their ordinal position also.
The sum of these multiplications modulus 11 subtracted from 11 is taken as the check digit (a result of 10 is translated to 0).
This scheme is similar to the ISBN 10 check digit scheme.