It was a relationship forged behind bars that led to artist and former inmate Nigel Milsom asking criminal barrister Charles Waterstreet if he could paint his portrait.

Waterstreet agreed and now the portrait, Judo house pt 6 (the white bird) has been named the winner of the 2015 Archibald Prize.

Accepting his award at the Art Gallery of New South Wales, a clearly humbled Milsom says Waterstreet was responsible for restoring his faith in the legal system.

In 2012, Milsom was sentenced to a maximum of six years in jail for armed robbery. He was serving that sentence in Cessnock Correctional Centre when he was awarded the $150,000 Doug Moran National Portrait Prize in 2013, before being released mid-last year.

Speaking before a large crowd of media, fellow Archibald finalists and Waterstreet himself, Milsom dedicated the award to his family and friends, thanking them and Waterstreet for supporting him during such trying times.

“My relationship with Charlie started to become fairly serious when he represented me in court,” Milsom says. “The painting is about when you’re in that particular situation, all your faith is placed into another human being and in some ways they become almost mythical in your imagination.”

Milsom’s oil on linen portrait is the largest of the 47 Archibald finalists, and depicts Waterstreet as a giant, almost fabulous figure complete with skeletal hands emerging from the dark background.

“It’s Charlie reaching out to say, ‘I’m here to help’,” Milsom says, describing the portrait as, “An attempt to depict him as a giant; part-man part-mythical creature, with hands that appear otherworldly.”

This is the third time Milsom has been an Archibald Prize finalist, and his first win. He has previously been the recipient of the Sulman Prize (2012) and is also the subject of a portrait by another finalist in this year’s Archibald exhibition, his friend Matthew Kentmann’s Nigel Milsom, Sunday 3 May 2015.

The runner-up for the Archibald Prize is Sydney artist Mitch Cairns for his portrait of fellow artist Peter Powditch.

Before naming Milsom as the 2015 Archibald Prize winner, the president of the AGNSW board of trustees Guido Belgiorno-Nettis also announced the winners of the prestigious Wynne and Sulman prizes.

Judged by former Archibald winner Lindy Lee, the winner of the $40,000 Sulman Prize was young Sydney artist Jason Phu for his work I was at yum cha when in rolled the three severed heads of Buddha: Fear, Malice and Death. The award recognises the best subject painting, genre painting or mural project in oil, acrylic, watercolour or mixed media.

The winner of the $50,000 Wynne Prize for the best landscape painting of Australian scenery or figurative sculpture was awarded to Melbourne artist Natasha Bieniek for her miniature oil painting, Biophilia.

The finalists of all three prizes are on display at the AGNSW until September 27 before being exhibited on a regional tour.

artgallery.nsw.gov.au/exhibitions/archibald-wynne-sulman-prizes-2015