"Whenever I see a Maori with a moko and an interesting face, I stop them and ask if I can paint their portrait," Mr Bronnimann said.
"I give them my business card and point them to my website so they know I'm serious."
Ms Robinson-Coles agreed to have her portrait painted, along with her sisters.
Mr Bronnimann said the portrait started with a photo shoot by photographer Albert Sword. It then took Mr Bronnimann around two years to complete the portrait - 412 hours in total. The portrait is made of oil on linen and measures 183cm wide by 122cm high.
This is the fourth time Mr Bronnimann has entered the awards. When the gallery phoned him to tell him he'd won he "thought the woman was joking".
"To say I'm pleased is something of an understatement," Mr Bronnimann said.
He intends to spend his $20,000 on a trip to Europe to see his parents and to visit the Meisel Gallery in New York.
Ms Waikerepuru said Mr Bronnimann had invited the three of them for a viewing of the portrait around two weeks ago.
"We were blown away. We're so appreciate of how he's captured our wairua [spirit] and our essence as wahine Maori."
Ms Waikerepuru said the three were "geneologically linked" and considered themselves to be sisters. They have strong iwi connections to Te Atiawa, Taranaki Tukau, Tangahoe, Nga Ruahine, Ngati Ruanui, and Nga Rauru.
"We are all great-grandmothers and have worked at varying levels supporting whanau, hapu and marae social, cultural and economic development within our tribal areas for about 30 years, while also pursuing our own individual creative endeavours that include performing ceremonial rituals and traditional and contemporary weaving."
Mr Bronnimann, Ms Robinson-Coles, Ms Waikerepuru and Ms Wihapi-Waikerepuru travelled to Wellington for last night's awards ceremony.
Ms Waikerepuru said she had been looking forward to the event.
"We're very happy the painting has won this award. For it to be shared and appreciated by a wide group is wonderful.
"We are honoured to be standing with Andre," she said.
+ See more of Andre Bronnimann's work at www.andrebronnimann.com or www.facebook.com/BronnimannArt.