An Anzac painting by Horace Moore-Jones’ of Simpson and his Donkey is expected to bring around $500,000 at a New Zealand auction.

An iconic painting once believed to be of Anzac legend John Simpson and his donkey is expected to sell for around half a million dollars.

One of British-born painter Horace Moore-Jones’ first depictions of a medic ferrying injured troops on a donkey during the Gallipoli campaign will go under the hammer in Auckland on July 22.

While named Simpson And His Donkey after the Australian soldier, the painting actually portrayed Richard Henderson, a New Zealand medic who took up the job after Simpson was killed.

International Art Centre director Richard Thomson said the painting was the most significant of Moore-Jones’ work as it shows the brutality and hardships endured by Anzac troops.

“The pain and suffering this painting depicts is only part of its very powerful and evocative symbolism of war,” he said.

“Of all the conflicts New Zealand has been involved in over the last 100 years or so, there is nothing like this.”

Mr Thomson said he believes the painting would bring between $NZ300,000 ($A267,606.26) and $NZ500,000 after another, smaller version by Moore-Jones sold for $NZ257,000 in March.

The painting was purchased from Moore-Jones’ widow for STG300 by the Commerce Club of Auckland in 1926. But the club said it needs to sell it off to afford essential repairs for its building.