The next instant her life changed forever.
A horror head-on crash claimed the life of the other driver, while Purnell was left battered after being assisted from the wreck by the jaws of life.
Her son Liam, aged just 1, had been sleeping when the crash happened.
"He has talked a few times about the accident, about mum being broken, and about how he never wants to be in an accident again."
He still panics when he hears the sirens of fire engines, Purnell said.
"We talk about how that is a good sign because that means someone is coming to help."
"It felt like minutes, it must have been seconds - because they said I wouldn't have had time to react at all," Purnell said.
"The accident did not happen in slow motion, but it was like my thought processes were happening in slow motion.
"I had so many thoughts at the same time."
Purnell said she knew she was not scared of dying but felt an overwhelming sense that if she did it was not going to be fair to her children or her husband.
"I do not remember the impact," she said.
"One of my first thoughts was that the car was on fire because it seemed all smokey.
"It felt like I was choking."
She was struggling to breathe against the gas from the airbags and because she had broken five ribs.
Purnell also broke her collar bone, fractured her right foot, and dislocated a big toe.
"I remember asking about the other driver as well," she said.
"I couldn't get my head around the fact that she died.
"I think people can forget how dangerous cars can be."
She emphasised to her children that they survived because they were wearing seatbelts.
The crash taught Purnell to look for the positives in everything, she said.
She was grateful she had people to take care of her after the crash - people to wash her hair and help her get about.
"I threw up on some orderlies, it must have been awful for them but I couldn't do anything about it. So I had to let go of some of the guilt of that as well."
She also experienced morning sickness as she recovered from the crash.
"I still couldn't believe it when he was born," she said.
"I just thought, 'How could he have survived that?'"
Her husband was also in disbelief after seeing the "severe bruising" on her stomach from the seatbelt.
For the couple Evan is a "miracle" baby - the only one of their three young boys that was conceived without fertility treatment.
She had taken a home pregnancy test the night before the crash and it was positive.
Purnell has recently been able to resume work in early childhood care. It is a passion that closely aligns with the joy she takes in raising her three boys.
She is also conscious of spending time with family.
"Everything can be taken away so quickly."
She said it also strengthened her appreciation of emergency services teams including the rural firefighters and the Auckland Westpac Rescue Helicopter team.
"They are really skilled people and they need more support to keep doing the good work they are doing."
The driver in the other car was a 27-year-old woman from Awhitu.
After investigating the crash, police referred her death to the Coroner.
A date is yet to be set for an inquest.
