Ms Mulcahy said she had considered herself retired from fashion design after a career that included owning her former Rotorua store The Design House.
But when she saw the flax images, she "instantaneously" regained her "mojo", she said.
The pair sat down to discuss their collection and came up with the idea of rainwear.
"[Rainwear] fitted with the flax - it used to be used as raincoats," Ms Mulcahy said.
She said the colours were not altered, coming out on the coats just as they looked under the microscope.
On Friday they headed to Wellington for the award finals and blew away the judges. As part of their prize, they will be part of the Miramoda showcase at fashion week in September.
Ms Mulcahy said they had a few tweaks to make in preparation for the Auckland show and the attention that will come with it.
"It's a pretty big deal for us - nobody knows anything about us," Ms Mulcahy said. "It's $25,000 worth of exposure .. . in front of international buyers and media, it's pretty cool."
She said they hoped to have the coats ready to sell by then and were already fielding inquiries from people wanting to buy one.
"They're classic coats, anyone can wear them," she said. "We've seen that we're on the right track, it's about selling them at the end of the day."
Meanwhile, Ms Whitewood said her win, with a collection based around fringed pig leather, was exciting and affirmed the work she had been doing.
She said she was now busy preparing for fashion week while working full time on her Rotorua-based design business.
Mitchell Vincent of Taupo won the Emerging Designer category.
