Climate Change Issues Minister Tim Groser has reprimanded a Maori leaders group over its representative at the recent United Nations meeting in Peru.
The letter has been labelled as "bullying" by the Labour Party.
Mr Groser has returned from talks in Peru where 190 countries agreed to a framework for pledges to cut greenhouse gases.
Chris Karamea-Insley is an adviser to the Climate Change Iwi Leaders Group and also travelled to Peru as part of New Zealand's delegation at the meeting on climate change.
In a letter to iwi leader Apirana Mahuika, Mr Groser appeared unhappy with Mr Karamea-Insley's open criticism of the New Zealand Government's climate change policy.
Mr Groser said in future, he would have to be more forthright in expressing expectations of good faith and respectful behaviour to all members of international delegations.
Mr Karamea-Insley told Te Manu Korihi news he would prefer Mr Mahuika to comment on the letter and he is expected to reply to.
In an interview before he left for the meeting, he said he had been working with officials and ministers about the iwi leaders' concerns they wanted addressed.