Kendal Briquel Levine, a 20-year-old American in Canberra on her Mormon mission had been hit by a car and was trapped underneath.

Police say the quick thinking actions of Mr Fearn, a 25-year-old tradie, may have saved the life of the missionary, who remains in a critical but stable condition in The Canberra Hospital.

"I heard something outside, didn't think much of it and then my mum screamed at the top of her lungs," he said.

"I ran outside and seen there was a girl underneath the car, I didn't really know if she was dead or alive at that point.

"I checked to see how far under [the car] she was, how heavily it was on her ... [then] I sprinted back to the house, grabbed a car jack, picked the front of the car up and rolled her out from under the car into the recovery position.

"She was making some sort of sound as soon as I lifted the car off her ... and I was relieved that she was actually alive."

He said Ms Levine threw up and started breathing normally once in the recovery position, but was still unconscious.

Mr Fearn, a commercial air conditioning technician, had done a first aid course as part of his technical training.

"I didn't know how I was going to help if I didn't get the car off her, so that's what my mind first went to, [it was] probably lucky that I thought of it so quickly," he said.

Mr Fearn's quick thinking has been praised by Ms Levine's parents, David and Melody, on Thursday night.

The couple have been by their daughter's side since arriving in Australia from Hawaii earlier this week.

Mr Levine said the family looked forward to meeting the man whose actions might have saved his daughter's life.

"Not only did he help lessen her injuries but he may well have saved her life," he said.

"Our family is very grateful and forever in his debt."

Ms Levine underwent surgery this week to releive pressure from her brain, resulting in the removal of a section of frontal skull to create room for her brain to expand. The pressure was stable as of Thursday.

Mr Levine said the family, including Kendal's older brother Dax, 22, had been overwhelmed by an outpouring of support from across the world.

"We have a fairly large extended family and friend base but this has been far beyond anything we could have imagined," he said.

"It's just unbelievable the people of all faiths who are sharing their heartfelt emotion and desire for our daughter to get better."

Mr Levine described his daughter as "selfless" and "very active" and said she had given up an American basketball scholarship to serve a mission for the Church of Jesus Christ of Latter-day Saints.

Mr Fearn's actions in using a trolley jack, which he had in his garage, to free Ms Levine were also praised by ACT Police.

"By stepping in and assisting, there's little doubt this man helped reduce the trauma on the woman," Acting Superintendent Rod Anderson said.

Ms Levine was with a friend and had been returning to her Toyota Corolla which was parked on the nature strip on William Hudson Crescent, some metres from the roadway, and had opened her driver's door to get back in the car.

A white Chrysler 300C, driven by a 26-year-old Monash man, collided with the driver's side of the Corolla, trapping Ms Levine under the Chrysler.

"[The driver] was still there, I think he was probably in shock, he didn't know what to do," Mr Fearn said. "Her friend was hysterical and holding her arm while she was still under the car when I got there.

After rolling her into the recovery position, he ran back to his house again to get a blanket to cover the unconscious woman until emergency services - his mother had made the call - showed up.

He said he has no idea how long it all took, but thinks it would have been "five minutes, absolute tops".

"When I had stopped running around I was buggered, sprinting back and forth."

His friend, Vinny Shankar, was amazed to eventually learn of his heroics, after Mr Fearn cancelled their planned visit to the gym that night.

"He's always been quick on his feet and he's always been a very helpful and considerate person, so it came as no surprise to me that he was able to think so quickly on his feet," Mr Shankar said.

"He's a pretty modest person ... he mentioned it to me on Friday night, but he talked about it in detail on Monday.

"He's told me it's not something you'd wish upon anyone seeing, but he's taken it quite well."

Anyone who may have seen the collision and who has not yet provided a statement to police is urged to do so by contacting Crime Stoppers on 1800 333 000, or via act.crimestoppers.com.au.

With Emma Kelly