A descendant of Hone Heke says he's delighted that what's believed to be a piece of one of the flagpoles his tupuna (ancestor) chopped down will go under the hammer in a week's time.

But mystery surrounds the authenticity of the find, valued at $20,000. The wooden piece measures 20cm by 9cm and an inscription on its silver plaque reads: "Piece of Flagstaff at Russell, Bay of Islands on which was English Flag cut down by Hone Heke in 1845 and was the cause of the New Zealand War."

The Ngapuhi leader was the first to sign the Treaty of Waitangi in 1840, but encroaching British authority over chiefly power saw the Kororareka flagpole cut down in 1844. Heke had given the timber for the pole in the first place.

It was replaced but chopped down again twice in January 1845, with fighting breaking out between hapu and British forces in early March. It was cut down for the fourth time on March 11 after a diversion was created by another Ngapuhi great, Kawiti.

Great-great nephew David Rankin said it was excellent to see a symbol of New Zealand's past resurface.

"Heke always said he had a right to chop those poles down because the trees were taken off his property.

"The flagpole was the biggest survey peg in the country, to him it was driven into the ground to state the ownership of this country by the Pakeha."

The sale was also timely as the tribe was going through the Waitangi Tribunal process.

"I think it's lovely that it's actually surfaced on the eve of Ngapuhi's claim. This thing is there to remind us about our claims process about our grievance," Mr Rankin said.

He hoped a New Zealand institution such as Te Papa would buy the piece.

Auction house Dunbar Sloane is selling the piece for Lady Caroline Simmonds, a granddaughter of the Earl of Ranfurly, who was the Governor-General from 1897 to 1904.

Dunbar Sloane jnr said much was still unknown about how the piece came into Ranfurly's possession, or which flagpole it was from.

Asked how he could be sure this was actually a flagpole fragment, Mr Sloane jnr said Lady Caroline held significant amounts of history from her grandfather's time as Governor-General.

"You go through the descent - there's impeccable history. It's not like someone walked in off the street and said 'I found this I want $20,000.' The plaque is very old."

Historian Paul Moon said there were doubts because of the elapsed time from the Northern Wars and when Ranfurly received it.