Mae Khong died in July at the age of 84, and the winning picture was taken at her home in the Thai province of Nakhon Nayok, depicting her bathing after a debilitating fall.
The winner was chosen from more than 1000 entrants and 30 finalists, and announced at the National Portrait Gallery in Canberra Friday evening.
Mae Khong would be surprised and incredibly proud to know the portrait won an award, Beard said.
"It's very touching to think of it in retrospect as a last gift ... the last contribution she would have to helping me as an artist," he told AAP.
The oldest of four siblings, Mae Khong represented a second mother figure for Beard, and as the last surviving family member of her generation she was also a cherished link to Thai heritage and culture.
Mae Khong migrated to Australia but returned to Thailand in the last years of her life, and Melbourne-based Beard visited regularly.
But his aunt never quite saw the point of all the pictures he took of her on those trips for his artistic archive.
"I think she had a hard time really understanding why ... I don't think she saw the worth or the value in what she represented," he said.
The winning image shows Mae Khong sitting naked on a blue plastic chair by a tub of water, facing away from the camera.
Beard says he and his aunt agreed on the composition, which allowed him to capture the moment but also respect her privacy.
The artist is actually best known for his surreal multimedia sculptures and installations, and generally uses photography to develop these works.
The National Photographic Portrait Prize started in 2007 in order to encourage contemporary portrait photography in Australia.
It's the last time the prize will be awarded, as the gallery is developing a new photography prize to run from 2027.
Beard will receive $30,000 from the gallery, and another $20,000 of equipment from camera maker Canon - which would have somewhat allayed his late aunt's concerns about making a living from art.
"She was always concerned about the financial stability of being an artist, always pushing about 'Why can't you just pivot to another career, something a little more stable?' " said Beard.
The first time finalist prize has gone to Phuong Nguyen Le for a portrait of his father Lê Ngọc Long, taken to mark 50 years since the end of the Vietnam war.
Hussein Abdirahmad Mohamud has previously been announced as the winner of the Art Handlers Award, for an image of Melbourne public housing estate resident Eedo/Aunt Khadija packing up her home.
A retrospective of the prize is being shown alongside the 2026 finalists exhibition from Friday until October 11 at the National Portrait Gallery, before touring nationally.
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