Re: Advice on NZ printers proofs.
Did you have any specific issues or material in mind?
There is quite a bit of De La Rue material from the 1960 definitive issue which is appearing on the market at the moment imperf, or in the case of the 3d Kowhai, three colours missing. In the latter case this (so called) error comes from a series of colour separations made by the printer in 1959 ahead of the first issue. Fine if sold as a set of colour separations but not if sold as a missing colour.
The 1963 Railway Centenary also exists as a series of colour separations. From memory these are imperf. Be suspicious of any single example of this issue being offered as 'spectacular' missing colours.