I believe the data base that shows all those NZ airstrips predates the openstreetmap project by many years. The airstrip information is part of Land Information New Zealand's survey data. Old 1;50,000 charts show very many airstrips, probably all the strips in the link above. These charts are viewable online by going to the LINZ website and zooming in on a selected location.
Almost certainly a significant proportion of the strips shown have been disused for many years. No maintenance will have been carried out with the result that the strips may be difficult to identify from the air and may have disappeared altogether. Equally it is likely some new ones have been set up. LINZ is a mapping operation not an aeronautical information service .
Last edited by Tagron; 14th Dec 2019 at 09:18.
Reason: typo