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Mr Williamson, who is a Maori mental health worker at Wakari Hospital, has been drawing with pencil since he was a child and this year decided to tackle oil paintings, starting with small works.
One of those works, based on a photograph, depicted the late poet Hone Tuwhare.
Mr Williamson had been "apprehensive" about undertaking a larger work, and in public, despite his inexperience.
But, after finding some helpful tutorials on the internet, he decided to "stretch" himself, to "push through that and get the best out of the experience".
At 10am on Saturday, Mr Williamson set up his easel on the North Ground and began painting the old building now occupied by the Good Earth Cafe.
"I was concerned that I might attract criticism - people might see me out here with a big canvas and I'm a beginner," he said.
However, he met a wave of positive response as many pedestrians, including overseas tourists, stopped to talk and asked about his artistic and Maori cultural background.
In addition, the cafe management gave him a coffee.
After working on the project for five hours on Saturday, he was back yesterday to finish it, and planned to do more.
"The public have been very positive. They see someone out painting and they want to know where you're from.
"It gives you confidence to go out and celebrate the environment in public settings."