"I just did it one day for the hell of it," Stewart said.
"When I got accepted, I thought, 'well, that's nice' -- then I saw I'd been chosen from 500 entrants and thought 'oh my God'.
"The painting obviously speaks to people."
Stewart painted her Parkin Prize entry in February while staying at Earthskin, a creative retreat at Muriwai, west of Auckland.
At the time, she was reading about McCahon, who had a bach at Muriwai.
She said her inspiration to paint McCahon was the sprawling coastal terrain of the area -- similar to that which would show up in his giant landscape works.
"Muriwai is expansive, vast and wild -- it's all beach and sky and sea. There's a real spirituality in that landscape."
Stewart's painting features McCahon at his bach, with one of his paintings, of a woman holding a lamp, in the background.
She used a predominantly black and white colour scheme, representing McCahon's fascination with light and dark contrasts.
The painting is also a nod to Stewart's passion for permaculture and food growing, referencing the "edge effect" -- where greater diversity of life is found in the edges of overlapping ecosystems.
"McCahon was always telling his students to look for the light in the spaces between, such as the edge where the sea and sky meet.
"In nature, the same applies -- the greatest life happens on the fringes."
Stewart's mode of creating is also based in nature -- she used encaustic painting techniques for Muriwai with beeswax and resin made from damar gum.
This same technique was used by the Egyptians to decorate their burial caskets.
"Acrylic paints are basically plastic, so this is more environmentally friendly."
Stewart trained in interior and textile design in her youth, but discovered fine arts while travelling in Italy, doing sculpture restoration in cathedrals.
She began painting while living in London, and has had several national exhibitions since returning to New Zealand in 2000.
Her World War I-themed mixed-media exhibition, Remember to Remember, is on display at Carterton Events Centre.