The text on the presentation pack issued on Wednesday, said Prince William was born on 21 May 1982. The correct date was 21 June.
New Zealand Post says the error was not picked up during the checking process and it is regrettable and embarrassing.
The company is asking people to return the product.
But a leading British stamp dealers, Apex Philatelic Auctions, says people should not return them because the mistake, and the small number of packs may increase their value.