The Waikato iwi claims the government's decision to pull back on the use of the Māori language in the public sector breaches its promise under the Treaty of Waitangi settlement, to protect and preserve te reo.
Spokesman Tukuroirangi Morgan said a case had been filed in the High Court.
"We are holding Winston Peters and David Seymour to account for the blatant disregard for the rule of law that continued to remind this nation that there is one law for all. These are the very two people who have breached our 1995 settlement and we will hold them to account and the only way we can do that is to take them to the High Court, which is exactly what we've done."
When a court date was set, he said, a train would be chartered to take people from Tāmaki, Auckland through the Waikato.
He believed several hundred people supporting the iwi would be in the courtroom.
However, Treaty Negotiations Minister Paul Goldsmith said the government took Treaty settlements seriously because the Treaty was a foundation of New Zealand and important for the country's past, present and future.
The government valued the Māori language and culture and would honour existing Treaty settlements, he said.
But he said it remained committed to the coalition agreements.
