She said she got in touch with chief medical officer of Norbert Kraft to confirm the selection was real.
Ms Flower, who found out about the competition on Reddit, said people were quite sceptical about it but she went ahead with the application anyway.
"I was not going to look back later and kick myself because I didn't apply for it," she said.
"If there's any chance that this could happen, then I want to be in on it."
She said her parents thought it was a joke at first but were quite shocked to hear she had made it through.
After the honeymoon period of the selection had worn off, Ms Flowers wrote a pros and cons list to pit what she would miss versus what she would gain.
She said she would miss her family and friends the most should she make the trip, but also little things such as walking her dog, Rooney, or spending time out in the sun.
"But I also think of what I'll gain," she said.
"Knowledge about our universe, which others and myself would not have without this opportunity."
Ms Flower, who grew up in Zimbabwe, said she had been interested in space and exploration since she was young.
"In Zim[babwe] we had the most amazing night skies and when we flew back in 2000 to New Zealand I remember looking down at Earth from my plane window," she said.
After making it through this latest selection round, Ms Flower needs to pass a medical test.
"I suppose when you send people to Mars you want to make sure they're at the top of their game," she said.
To get to that level, Ms Flower has been studying the effects of space on the human body, continuing with her nursing studies and getting fit.
"I'm 100 per cent committed," she said. "I'm willing to give up my comfortable life."
