An embarrassing mistake has forced NZ Post to withdraw its royal wedding presentation stamp pack only a few weeks before Prince William, the second in line to the British throne, and Kate Middleton are married.

The pack, which sold for $19.90, was released this week but included the wrong birth date for the prince in the text.

Instead of the correct date of June 21, 1982, the pack said Prince William was born on May 21.

The mistake was not on either of the two $2.40 stamps of Prince William and Kate Middleton released yesterday by NZ Post but in the text of the presentation pack.

NZ Post spokesman John Tulloch, said it was a regrettable mistake and the unsold packs would be withdrawn and reprinted.

He said anyone who had bought the packs and wanted them replaced should call 0800 782 677 to arrange a replacement.

"We used an external writer to do the copy and we have a checking process. That did not work. We are embarrassed by it," said Mr Tulloch.

Steven McLachlan, president of the New Zealand Stamp Collectors Club said the mistake would not mean the packs would be worth a lot more money but it was very embarrassing for NZ Post.

"It might worth $40 or $50. It is something they should have got right but they mucked it up."

He said it was "pretty slack" for an organisation like NZ Post to get it wrong when they had so many resources.

The presentation pack included a stamp set, a miniature sheet and stamp first day cover.

On its website NZ Post said the pack was "temporarily unavailable. Please try early next week."

It also said the Royal wedding was the wedding of the year and would be a truly special event, uniting nations in a celebration steeped in history, tradition, pomp and pageantry.