Image caption Uncle Paddy was admired by judges for its delicate brushwork

A prominent Australian artist who was jailed earlier this year for robbing a convenience store while high on drugs has won a major national art prize.

Nigel Milsom was awarded the Doug Moran National Portrait Prize, winning $150,000, for his portrait Uncle Paddy.

He pleaded guilty to robbing a 7-11 store in Sydney in April 2012. He later apologised to police, saying he thought he was "buying doughnuts".

Milsom, a former security guard, is serving six years for armed robbery.

The artist, who suffers from depression, completed the painting earlier this year while on bail awaiting sentencing for the crime.

The winning portrait was picked by 2009 Moran Prize winner Ben Quilty and ex-Art Gallery of South Australia director Daniel Thomas, who were not told the names of the artists.

"The fact is it was the best painting," Quilty told the Australian newspaper. "I judged the painting, not him."

"Even someone who has gotten into some state of punishment for whatever he did could come good," said judge Daniel Thomas . "So that's how I reconciled myself with Milsom getting that money."

He praised Milsom's "extraordinarily refined technique, the brushwork, so smooth, so delicate".

Image caption Milsom with his painting Judo House part 4, which won the 2012 Sulman Prize

Uncle Paddy depicts one of Milsom's grandfather's oldest friends; Thomas called it "a very old-fashioned Australian face".

"There is a quiet sadness about Paddy that seems to stem from the realisation that death will be visiting him soon, too," said Milsom.

"When I painted his portrait I got a sense that he has learnt to sit with this feeling of sadness, which has given him a greater strength and wisdom."

The Moran Prize, which annually awards contemporary Australian portraiture, is now in its 25th year. All the winning pictures become part of the Moran Arts Foundation Collection.

Last year Milsom won the $30,000 art prize for the 2012 Sulman Prize. Weeks later, armed with a knife, axe and a fake gun, he and another man threatened to kill a shopkeeper while holding him hostage in the store room of a 7-11 store, reported the Telegraph.

"My state of mind is hard to describe ... I thought I was buying doughnuts,'" he told the court earlier this year. "It felt like I was watching something as a movie that wasn't really happening."

At the time of the crime, the court heard the artist had recently lost his mentor, art curator Nick Waterlow, and his sister, and was high on a combination of heroin, crystal meth, prescription drugs and alcohol.