Representatives of more than 80 Māori tribes have issued a rare plea to King Charles III requesting his intervention in New Zealand politics, amid growing tension over the government’s policies for Māori and a souring of the relationship between Indigenous people and ruling authorities.

The National Iwi Chairs Forum – a collective of tribal leaders – has sent an open letter to the King asking him “to ensure that the [New Zealand] government does not diminish the crown’s honour” over what they believe to be ongoing breaches of the crown’s promises made to Māori in the Treaty of Waitangi, the founding document of New Zealand.

Since it took office last year, New Zealand’s rightwing coalition government’s policy direction for Māori has sparked the biggest ever protest over Māori rights, mass meetings of Māori leaders and condemnation from the Waitangi Tribunal, an institution that investigates breaches of the Treaty of Waitangi. The treaty is an agreement signed in 1840 between more than 500 Māori chiefs and the British Crown and is instrumental in upholding Māori rights.

The forum has provided the Guardian with first access to the letter, which is signed by more than 500 people, including tribal leaders, representatives of Māori organisations and others.

The chair of Ngāti Wai tribe and chair of one of the forum’s groups, Aperahama Edwards, told the Guardian that Māori are tired of their concerns going unheard.

“We’ve had 184 years of pandering to the decency and goodwill of the government and it is not reciprocated, so the thinking here is to bring [these issues] to the attention of King Charles with the hope he can intervene.”

The rationale behind many of the government’s proposals is to end “race-based” policies, tackle crime and reduce bureaucracy. The coalition has said it is committed to improving outcomes for Māori and all New Zealanders.

But critics fear its policies, including rollbacks of the use of Māori language in public services, the dismantling of an institution designed to remedy inequities in health, and the introduction of a controversial bill that seeks to radically alter the way the treaty is interpreted are undermining Māori rights, igniting anti-Māori rhetoric and eroding the Māori relationship with the crown.

Māori make up 20% of New Zealand’s population and face disproportionally negative outcomes in health, housing ownership, employment rates, education and prison numbers.

Margaret Mutu, a forum chair and professor at the University of Auckland who helped write the letter, said she was concerned the coalition’s policies were a serious violation of the treaty. She hoped the King would remind the government of its treaty obligations.

The two-page letter begins with an acknowledgment of the King’s relationship to the late Māori King Tuheitia and a hope the relationship will continue to thrive under the new Māori Queen, Nga wai hono i te po.

It references Queen Victoria’s negotiations with Māori chiefs to establish the treaty in 1840 and the crown’s breaches of those promises in the years since. It says in recent years the relationship between Māori and the crown had been developing well but that had changed under the new government.

The letter claims the new coalition government “has promised to attack” the treaty and the rights of Māori.

The letter lists legislative changes that are causing the greatest concern, including putting up barriers to revitalise the Māori language, policies that could further disconnect Māori children from their ancestry if they are taken into state care, reducing the function of the Waitangi Tribunal, putting the land and sea at risk of mining and more.

The letter points to a speech the King made at the Commonwealth Heads of Government meeting in 2022 where he discussed a need to “forge a common future that benefits all our citizens” and “find new ways to acknowledge our past” and appealed to the King for help.

“As a constitutional monarch of the crown and a descendant of Queen Victoria, we seek your intervention to ensure that the government does not diminish the crown’s honour,” it said, adding the signatories are united in their “grave concerns about what these actions will do to our whānau [families]”.

Edwards said the obligation to honour the promises of the treaty rests not only in the descendants of the chiefs that signed it, but in the descendants of Queen Victoria: King Charles III.

“We believe that his environmental and social justice concerns and influence can play a crucial role in reminding the government of the importance of upholding the sacred agreements of [the treaty],” Edwards said.

New Zealand’s minister for Māori Crown Relations, Tama Potaka, has been contacted for comment.

Buckingham Palace was also contacted for comment.

Edwards said the forum had yet to hear back from the palace, but he hoped the King would read the letter and offer some support because the wellbeing of Māori and the treaty relationship was at stake.

“We’re turning to some of the saddest times experienced in our country … we will not sit by and be complacent.”