The 1921 painting titled Portrait Of Mary Lushington Bernard D'Oyly, which has been in the sitter's family in Devon, England, for generations, fetched Stg 10,000 ($A18,600).
They had always believed it was a real Roberts, but it wasn't verified until a few weeks ago by former National Gallery of Australia senior curator Dr Mary Eagle.
Roberts' mature style was so distinctive that Dr Eagle instantly knew the portrait was authentic when she saw an image of it.
"It's a very handsome painting," she told AAP.
The painting was auctioned on Wednesday in a sale of Old Master paintings and European Art by auctioneers Dreweatts, selling at the bottom end of the estimated price range, not including a 26 per cent buyer's premium.
While some of Roberts' paintings have sold for almost one million dollars, his landscapes have generally been more popular than his portraits on the secondary market.
Dr Eagle dug into research she had carried out in the 1980s to unearth a transcribed letter and sketch by Roberts that showed the painting was the real thing.
"The painting is exceptional in that there is a straightforward set of evidence showing its provenance," she said.
"Many of the major works of art around the world, a great many of them have no absolute provenance, whereas with Tom Roberts in that instance it was easy."
The evidence showed Roberts was in the village of Combe Raleigh, near Honiton in Devon, in the third week of August 1921 to paint a portrait of a Mrs B D'Oyly.
In a letter to his wife held by the State Library of NSW, Roberts included an ink sketch of the portrait showing its composition and noted "the colour is a good pale blue on Gold B'ground".
He wrote the painting had not been progressing happily but he had since "got hold of it" and it was going well.
Mary Lushington Bernard D'Oyly was "very spring like & lace & pearls", Roberts said, although the proponent of plein air painting said he felt working indoors was restrictive.
"There is much difficulty in working in an ordinary room - one gets jammed up, the light deceives one - when the place is not spacious & you are right against the window," the letter reads.
It is rare to unearth a previously unknown work by an artist as important as Roberts, according to Dreweatts' Anne Gerritsen - who was pleased Dr Eagle was able to connect the portrait with the letter and sketch.
"She immediately connected the dots and made that link, so that was brilliant," Ms Gerritsen said.
Roberts was born in the UK in 1856 and became one of the founding members of the Heidelberg School, which was at the forefront of the development of Australian Impressionism.
In 2017, another Tom Roberts initially thought to be fake was authenticated on the BBC TV program Fake or Fortune.
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