The oil painting of Roberts' friend, the painter and etcher Louis Abrahams, will go on public display for the first time when it features in the gallery's retrospective of the artist for its summer blockbuster exhibition.

Director Gerard Vaughan said the identity of the "generous donors" will be revealed when Prime Minister Malcolm Turnbull opens the exhibition in early December.

Roberts painted Abrahams in Melbourne in 1886 and the canvas had remained in the Abrahams' family until it was recently offered for sale by Christie's London.

"It's a totally new discovery, this is a painting that's never before been recorded," Dr Vaughan said.

"It's a great acquisition for the nation and for the people of Australia that this painting has now come home."

Dr Vaughan said the gallery had to move quickly to buy the painting when it came up at auction and "a number" of the gallery's friends and supporters had agreed to fund the purchase.

"We put all our resources into it ... there was a lot of interest from private collectors as well so that's why we felt we had to throw everything at it and fortunately we were successful," he said.

"Everyone in the Tom Roberts' study area was amazed, you think you know everything... and suddenly one that's never been recorded before turns up, it's very exciting."

The retrospective's curator Anna Gray said the painting's "rapid, free brushwork" displays the strong influence of modern French art to which Roberts was exposed when he spent time in Parisian studios shortly before returning to Melbourne in 1885.

"European portrait painters who were most influential on Roberts' development include Manet and Whistler,' she said.

Dr Vaughan said private support was crucially important to allow galleries to acquire work as annual government funding only covered operating expenses.

The NGA's acquisition was announced one day after the Art Gallery of NSW revealed two separate record-breaking bequests worth more than $21 million.

Dr Vaughan said galleries did not compete to acquire private support and he believed the philanthropic sector was growing with the NGA fortunately bequeathed a collection worth $8 million from retired ACT economist Alan Boxer in early 2015.

"I've never felt the philanthropic cake was limited, if one person gets a very large slice of that cake it [doesn't] really take away from the others," he said.

"I think the cake gets bigger and bigger and every time someone does something generous for us, NSW, Melbourne or one of the regional galleries I think it improves the whole context of giving for everyone in the country."

Abrahams was a lifelong friend of Roberts who he met at art school.

Both continued to paint together until Abrahams was persuaded to give up painting to take over his family's very successful importing business.

The painting is Roberts' only known portrait of his friend, but Abrahams did feature with his back to the viewer in Roberts' landscape, also owned by the NGA, A quiet day on Darebin creek a year earlier in 1885.

He was also the central figure frying eggs in The artists camp 1886. Both paintings will feature in the exhibition.

Roberts was a witness at Abrahams' wedding, and painted a portrait of Mrs Abrahams, now in the collection of the National Gallery of Victoria, as a wedding present two years after painting his friend.

Abrahams played a pivotal role in the 9 by 5 Impressions exhibition - Australia's first exhibition of "radical art" - by providing the cigar box-lids that many of the artists, including Roberts, painted on.

The 9 by 5 inch box-lids became synonymous with the Australian Impressionist movement, Dr Vaughan said.

"Tom Roberts was Australia's pre-eminent portrait painter in the late 19th and early 20th centuries," he said.

"This arresting and important work greatly enhances the NGA's collection of Roberts' portraits."

Tom Roberts opens to the public on December 4 until March 28. Tickets are on sale through Ticketek.