Dutch entrepreneur Bas Lansdorp's crowd-funded concept, which has drawn scepticism from space agencies including Nasa, involves settling the candidates in a permanent base on Mars, 225 million km from Earth, by 2025.
Mars One makes for one of the most ambitious reality television events ever, with successful applicants for a human colony being broadcast around the world.
Ms Fahey discovered she was on the shortlist while checking her emails before work a few days ago.
"I kept reading and thought, wow, oh my, I got through."
She believed her skills as a qualified horticulturist may have boosted her chances, but suspects it was her eagerness that played a bigger part.
"From what I can gather, I think they want more people that can work with people as opposed to qualifications, because they can always train you," she said.
She had long been ridiculed for wanting to be an astronaut, a dream she's had since childhood. Now it was beginning to look like the joke was on everyone else, she said.
"I posted on Facebook that I'd got through and my mother said, 'I'm not sure if I'm supposed to like that or not," she said with a laugh.
Ms Fahey still has to compete against 1058 candidates from 107 countries.
Organisers were not choosing candidates who did not take the mission seriously, and some had been culled after submitting videos featuring themselves naked.
Former Shortland Street actor Karl Burnett - who joked that he wanted to be the first human to go to the toilet on another planet and be installed as an evil dictator - hasn't heard back.
While the sci-fi fan told the Herald "it would have been pretty cool" to have made it to Mars, he admitted he'd only been "0.1 per cent serious" when applying.
Temperatures on Mars are comparable only to Antarctica, while no water, an atmosphere comprising 95 per cent of carbon dioxide and almost zero oxygen would make unassisted existence impossible.
Boldly going
* 200,000 Original applicants to Mars One
* 1058 applicants chosen from 107 countries
* 46 picked from Oceania (43 from Australia)
* 2025 deadline for the first Mars One colonists to arrive on the planet
* 225 million km: distance from Earth to Mars
* 38% the relative proportion of gravity on Mars to that of Earth
* -87C the low extreme end of mean surface temperature on Mars
* 950,000 parts per million: the amount of CO2 in the Martian atmosphere, compared to about 395 ppm on Earth. CO2 poisoning in man begins at about 1000 ppm.