"This is the most significant of all Horace Moore-Jones' work because of its size and the fact that it was probably the first he did which showed the brutality and suffering of a campaign which failed miserably but which also has shaped two nations, New Zealand and Australia, in the spirit of Anzac which endures today," Mr Thomson said.
"It is an extremely important and highly significant painting. Of all the conflicts New Zealand has been involved in over the last 100 years or so, there is nothing like this. There is nothing that shows such pain and suffering. It is a gutsy and dramatic portrayal of what war can do.
"But the pain and suffering this painting depicts is only part of its very powerful and evocative symbolism of war."
Moore-Jones died a hero in 1922 after he was badly burnt rescuing people from a hotel fire in Hamilton.
He never learned that his painting, which he originally named after John Simpson Kirkpatrick, an Englishman who enlisted in the Australian Army, actually portrayed the Kiwi, Henderson.
The painting is owned by the Commerce Club of Auckland, formerly the Commercial Travellers Club, which bought it from Moore-Jones' widow in 1926 for ?300.
It hung in the club's Remuera clubrooms for many years before it was loaned to the Auckland War Memorial and Museum where it has been an integral part of the Scars on the Heart exhibition which covers New Zealand's involvement in wars, from the Boer War which began in 1899 in South Africa, to recent United Nations deployments.
Anton Coetzee, the Commerce Club general manager, said the club needed funds for essential repairs and maintenance to its Remuera building and members had decided with great reluctance to sell the painting.
Mr Thomson estimated the painting would bring between $300,000 and $500,000.
The painting was likely to be protected under the Protected Objects Act, 1975, which meant it could not be taken out of New Zealand without ministerial approval and that was given only in exceptional circumstances, he said.
"This has got real guts and we think it is a national treasure," he said.
"It is a hugely important part of our heritage and in our view it should not be lost to the country."