Sir Don McKinnon delivers his lecture 'Dark Clouds Over Democracy' as part of the Commonwealth Parliamentary Association Lecture series, in Wellington, 2 May 2017.

Sir Don McKinnon (file photo) is a foreign minister and deputy prime minister of New Zealand and former secretary-general of the Commonwealth. Photo: Office of the Clerk of the House of Representatives (NZ)

The letter was sent to Mr Trump before he left for a trip to Europe where he's expected to have tense talks with the leaders of NATO member nations.

Mr Trump has accused NATO countries of underfunding the security alliance and unfair trade with the US.