Sand: crushed rock, shells etc smaller than 2mm in size
Data Dictionary for sand_poly: https://docs.topo.linz.govt.nz/data-dictionary/tdd-class-sand_poly.html
This layer is a component of the Topo50 map series. The Topo50 map series provides topographic mapping for the New Zealand mainland, Chatham and New Zealand's offshore Islands, at 1:50,000 scale.
Further information on Topo50: