The painting was only ever meant to pay for his ticket to London.
Instead, Henry Christian-Slane's portrait of his cousin has been declared the best in New Zealand for 2014.
The 23-year-old is the youngest artist to win the New Zealand Portrait Gallery's prestigious Adam Portraiture Award.
Speaking from London yesterday, Christian-Slane said he was left in a state of shock after his parents broke the good news.
"It's just wow, it's just amazing."
When he did the portrait, he was still living at home in Auckland, spending 11 hours every day working in a shabby, cold, central city studio crammed with other artists.
While scrounging for money to travel to Britain, his grandfather, former privacy commissioner Sir Bruce Slane, offered to commission three family portraits to help fund his ticket.
He entered only one of the paintings, on his grandfather's urging, just before he left the country late last year.
"But when you enter these competitions you never really expect to win. I just submitted it and forgot about it," he said.
He was particularly happy with the stark blue-grey image of his cousin Tim.
Award judge Nicola Kalinsky, director of the Barber Institute of Fine Arts in Birmingham, Britain, said Christian-Slane's portrait was simple but with a striking depth.
"The more I look at it the more I enjoy it. It is very direct and simple."
She said there was incredible range to the 61 finalist portraits at the Wellington gallery and amazing work from some as young as 15.
"It is pretty extraordinary for such a small country."
While Christian-Slane painted his cousin, other contestants chose more well-known subjects, including the late artist Ralph Hotere and celebrated ballet dancer Sir Jon Trimmer.
With two parents who are artists - his father is cartoonist Chris Slane - it was hardly surprising that Christian-Slane took to art.
He is now studying in London, living in a warehouse with 14 other artists and working as an illustrator for design firm Unit9.
His family will accept the award on his behalf. He said he would use his $20,000 prize money to give him more time to create art.
"I just want to do more of what I've been doing, really."
The Adam Award and Exhibition is on show at the New Zealand Portrait Gallery at Wellington's Queens Wharf from today till May 25.
- The Dominion Post