She and Mittendorff originally planned to import portable coffee machines to sell but couldn't find any that had all the features they wanted - like being able to boil water, grind beans and froth milk - so they decided to develop their own.
"Coffee makers have been made before and patented that are portable but there's no coffee maker that's ever been invented or had a patent registered that makes coffee like an espresso machine would," Ellin said.
"I thought, 'Why don't we just go for it? What's holding us back? We're young and have our whole lives ahead of us and have nothing to lose. We may as well just give it a go'."
The Whangaparaoa couple had both worked as baristas.
"We thought the idea of having coffee wherever you are without having to pay $5 for each cup of coffee would be good.
"The demand for coffee is so high, especially in New Zealand, so we sort of anticipated quite a good response to it, I guess, provided we were the first ones to do this," Ellin said.
More than 40 per cent of Kiwis surveyed by Canstar Blue in September said they spent less money since buying a coffee machine and 27 per cent preferred their homemade brew to what they could buy from a cafe.
The 9cm by 32cm PortaPresso has separate compartments for water, milk and coffee beans. It has a built-in grinder and uses battery-generated pressure and a CO2 canister to pump water through the compacted grounds and froth the milk.
The machine has an LCD screen and can make different coffees such as cappuccino, lattes, flat whites and long blacks. The temperature and grind settings are adjustable, Ellin said.
"It's really easy [to use]. If we get enough funding we're hopefully going to have it integrated with an iPhone app."
The couple have set up a Kickstarter to fund production but hope to sell them through international distributors and camping shops.
Ellin said they would sell the machines for about $320, but the first machines would be discounted to $240.
An Instagram page she and Mittendorff set up last week had attracted more than 10,000 followers and people were already asking to buy PortaPressos.
Rab Heath, a 29-year-old who co-founded technology startup Haptly, said it was "epic" other young Kiwis like Ellin and Mittendorff were focusing on user experience when developing new products.
"Coffee is the second-most-traded commodity in the world. But a world-first in a huge market only matters if people want it."
Andy Hamilton, chief executive of The Icehouse not-for-profit that helps Kiwi entrepreneurs with their businesses, said it was inspiring the couple had come up with an idea and "made it happen" so young.
Their age was not a barrier to success as long as they had the right support network, Hamilton said, and the next step for Ellin and Mittendorff would be to build up their business.
