"At this time ... staff of the TRIAON (Te Rūnanga-ā-iwi ō Ngāpuhi) Group will continue with daily business responsibilities as usual,'' chief executive Lorraine Toki said.
Tau's role is likely to be filled by deputy chair Mere Mangu in the interim.
As well as heading an iwi with an estimated 125,000 members, Tau was the deputy chairman of Tūhoronuku, the group which had been recognised by the government as holding the mandate to negotiate a Treaty settlement of behalf of Ngāpuhi.
That settlement process has, however, been stalled for years.
Tau's departure could allow a fresh start to New Zealand's last big Treaty settlement, likely to be worth hundreds of millions of dollars.
A new mandate proposal was put to hapū last year but voted down.
Rāniera "Sonny" Tau has resigned as Ngāpuhi chairman. Photo / John Stone
The leaders of the main group opposed to Tūhoronuku's mandate, Pita Tipene and Rudy Taylor of Te Kotahitanga, weren't commenting today and knew nothing more than was in the rūnanga's statement.
Also not commenting were Te Tai Tokerau MP and Minister for Māori-Crown Relations Kelvin Davis, who said it was a matter for the rūnanga, and Minister for Treaty Negotiations Andrew Little.
The 66-year-old Tau is nothing, however, if not a survivor.
He was previously forced to step down after an incident involving protected wildlife but regained the chairmanship a year later.
In 2016 Tau was convicted of killing kūkupa, or native pigeons, and conspiring to pervert the course of justice by shifting the blame onto someone else.
Tau was charged after five frozen kūkupa were found in his luggage at Invercargill Airport on June 16, 2015.
He admitted possessing them but said his daughter's partner had shot them. Later, both men admitted concocting the story and were charged with perverting the course of justice. Tau was also charged with killing the birds.
He was fined $12,000 for shooting the birds and ordered to pay the Department of Conservation $12,500 towards its costs. The judge also imposed 100 hours of community work and ordered the forfeiture of his rifle.
On the charge of perverting the course of justice Tau was sentenced to three months' community detention.
In August this year Tau faced the first serious challenge for his seat on the rūnanga board when Broadwood teacher and former Mana Party candidate Clinton Dearlove stood against him in an election to represent the Hauaruru takiwā (the district covering Kaikohe and Ōtaua).
Tau was re-elected to the board with 567 votes to Dearlove's 398. It was one of the highest turnouts seen in a takiwā election.
Rūnanga board members had been due to vote on the chairmanship next month.
