A rare and valuable painting by Colin McCahon could sell for a record price when it goes up for auction next month.

Photo: Supplied.

McCahon's The Canoe Tainui is being auctioned off as part of the private collection of the former New Zealand ambassador to the United States, Tim Francis, and his wife Sherrah.

Mr Francis was the former New Zealand Permanent Representative to the United Nations before becoming the Ambassador to the US during New Zealand's declaration that it would be nuclear free.

He died earlier this year, just two months before his wife, both at age 87.

The couple left behind a collection five and a half decades in the making, which has rarely seen by the public.

It includes up to eight McCahon paintings, three works by New Zealand artist Rita Angus and many more masterpieces.

Art+Object auctioneer Ben Plumbly said the pair collected in a way not seen anymore, which made the sale an important moment in New Zealand art history.

"I think it is without question one of the most important private collections of New Zealand art ever assembled - if not the most important."

"Tim and Sherrah, especially late in their life, were very private people so whilst a lot of people have heard of this collection it's very much the stuff of legend. Very, very few people have actually seen it so there's a great process of uncovering and revealing largely extinct phenomenal collection, phenomenal body of work that very few people have seen or that's ever been seen publicly.

The Canoe Tainui is an eight panel painting, which stretches almost five metres and was originally part of an exhibition in 1969.

McCahon became interested in Māoridom and painted the piece around the time of the birth of his grandson, who was of Tainui descent.

"He creates this kind of visual ode to the genealogy of the Tainui tribe in classic McCahon fashion. It's done on a very inky dark black background, the text is beautifully inscribed and written very subtly in many different shades of white and ocre and cream and it traces the genealogy of the Tainui tribe.

Mr Plumbly said the auction of the piece would be "uncharted territory"

"Normally a work of this significance is sold privately so we're hoping at auction it will bring to the mid to high one million. So anywhere between 1.4 and 2 million dollars."

He said no painting in New Zealand had sold at public auction for that amount of money.

"So it's going to be fascinating to see how the market treats with it and whether it really grasps the significance and is in a position to pay what will effectively be a record price.

"Look I'd love to think it would go to a museum and in many ways it's like a precious piece of taonga. It could and probably should belong in an institution but there is of course a whole set of economic pragmatics around a work of this significance and this price. So the simple fact with an auction is it will go to the highest bidder but it would be lovely to think it would go to a museum.

The painting will be exhibited alongside 214 others, as well as up to 300 pieces of ceramic and a large collection of books from the Francis' reference library, at the Art+Object gallery in Auckland from 1 September.