Background
Important Note: Initially, funnily enough it was the Death Index to which I turned for details of date of birth. Following on from an extensive trawl through the Auckland War Museum Cenotaph database, I decided to work my way through the Birth index, (what I believe to be the more reliable source for verifying date of birth). I have entered all births found covering the period 1 January 1890 to 6 July 1924.
Remember what I said about a birth having to be over 100 years old. Over this next few years you will see more and more dates of birth appear. I am going back through the roll every fortnight or so, looking for the latest lot of births which can be accessed through the Birth Index. You can visit the Birth Index here.
Verifying Date of Birth:
In some instances a search may produce no result. This could be for a number of reasons including: the person may have been born overseas, the birth may not have been registered, (quite common I found with a number of the Maori names), or the information regarding birth shown on either the Cenotaph record or in the death index is not correct, or not correct in so far as that it does not match the entry in the registry.

New Zealand nurses in a cemetery, during the withdrawal from Greece, World War 2. Evening post (Newspaper. 1865-2002) :Photographic negatives and prints of the Evening Post newspaper. Ref: PAColl-7796-87. Alexander Turnbull Library, Wellington, New Zealand. http://natlib.govt.nz/records/22448626
“Permission of the Alexander Turnbull Library, Wellington, New Zealand, must be obtained before any re-use of this image.”
Let’s work through a couple of examples.
Using William Robert Garton as our first we see quite a lot more information available when doing a search of the BMD Birth Index. Again it’s the same procedure entering his date of birth in the ‘search from’ and ‘search to’ date fields and then clicking the ‘Submit Search’ button. Now we have even more information showing up, namely his father’s and mother’s given names. We also see the birth registration number and again can place an order for a copy of the certificate if we wish.
Let’s do another one which has just year of birth: Richard John Howie shows as having died in 1966 at the age of 61. This means he was born around 1905. Doing a search from 1/1/1904 to 31/12/1905 shows he was in fact born either in 1904 or 1905 and the registration number is 1904/4854. Again, using the same refining technique we use with a death search, we find he was born on 12th August 1904.
It goes without saying of course that you will not be able to pull up details of men and women born overseas.