The trust bought its first painting in 1962 when four works by English watercolourist, John Barr Clarke Hoyte. Sir James Fletcher II paid $300 for the four paintings, of rural scenes and the town of Coromandel.
"It was the start of a corporate collection which has grown to more than 550 works of art valued at many millions of dollars and representing a very diverse range of New Zealand art and artists," a statement on the sale said.
The highest valued work on sale is Ralph Hotere's Port Chalmers Painting No 8 1972 ($70,000 to $100,000) but Mr Thomson said it was rare to have a Raymond McIntyre (self portrait, $40,000-$60,000).
While much of the art has been at Penrose, some has also been at Government House in Wellington and Auckland and other works were loaned for exhibitions and tours.
John Gow, a founding director of Gow Langsford Gallery who bought more than 100 Fletcher collection paintings, is sorry to see it being broken up but said he understood why.
"I would by lying if I said I didn't feel a little twang of emotion around the idea of it changing but I'm also philosophical," Mr Gow said.
Fletcher had by far New Zealand's biggest corporate art collection, followed by BNZ, he said.
Mr Fletcher said proceeds would be used to foster the trust's philanthropic aims for education and youth development.
Mr Thomson said there had already been a lot of interest in the sale and a 116-page catalogue has been printed.
"This is something the country has not seen before. This offering represents such a wide range of art that is particularly relevant and important in every aspect of New Zealand life from our very early days of colonial settlement to the present day. We are particularly excited about the sale," he said.
Works to be sold by: Gretchen Albrecht, Patricia France, Milan Mrkusich, Pat Hanly, Rhona Haszard, Ralph Hotere, John Kinder, Raymond McIntyre, Richard Killeen, Evelyn Page, Geoff Thornley, Alfred Sharpe, Michael Smither, Ian Scott, Gordon Walters, Sir Toss Woollaston.
• Auction of 77 Fletcher Trust works, 6.30-pm, Wednesday September 10, International Art Centre, 272 Parnell Rd. Available for viewing from 9am, September 3.