On the outside she was calm. Silent and focused - just like she is before a BMX race. She stayed like that for four and a half hours.

On the inside her body was a mess. A broken nose, broken sternum, two collapsed lungs and dangerous bleeding around her heart.

Then she heard the quivering voice.

The paramedic in the ambulance was calling ahead to The Canberra Hospital to prepare them for Buchanan's arrival.

"I was running on adrenaline. About 30 minutes outside of Canberra the ambulance guy called the hospital and he had a shaky, nervous voice," Buchanan said.

"If you're a paramedic you've seen a lot of things and they handle themselves so well. I thought if he's nervous ... that's when I knew it was life-threatening."

Buchanan had an "eerie" feeling before going to a private property 45 minutes outside of Cooma on December 30.

The two-time Olympian and eight-time world champion was at a friend's farm when an off-road buggy rolled and caused the injuries.

The driver was OK but Buchanan was in debilitating pain and struggling to breathe. She knew something was seriously wrong and that's when she started to fight.

Her fiance, American BMX rider Barry Nobles, saw Buchanan running towards him with tears in her eyes and blood dripping from her nose.

Knowing they had no phone reception, they did the only thing they could. They strapped Buchanan into the front seat of a car and started driving towards Cooma Hospital.

"I couldn't talk because I knew I would panic. So I went into a meditation of breathing ... if someone was talking to me I didn't hear them. If I panicked, that would have put extra pressure on my organs and who knows," an emotional Buchanan said speaking about the accident for the first time.

"In those situations, you've just got to fight. You've got no option.

"I'm doing rehabilitation now [at the AIS], but my head space is focusing on the human aspect of it rather than the athlete."

From the moment they arrived at Cooma Hospital, it took another four hours to get to Canberra via ambulance, where they drained 2.5 litres of blood and fluid from her lungs over three days and she spent four days in the intensive care unit.

For someone whose racing career is defined by milliseconds, a five-hour ordeal and week in hospital must have felt like a lifetime.

Buchanan says the only way she can describe the pain of collapsed lungs is the sensation of being winded but not being able to get your breath back. That's how she felt for five hours.

But despite her still-obvious discomfort - even now she has to roll herself in and out of bed because of the pain - Buchanan is framing this as a positive story.

One of rebuilding her body, instilling a belief she can overcome any adversity and, remarkably, helping her push boundaries on her bike

"I'm happy that situation wasn't any worse and I've got opportunities to continue my career and ... to just live," Buchanan said.

"When life is so fast, you don't always appreciate little things. I remember sitting in hospital and watching the sunset. It's just genuine gratefulness.

"I know I survived that accident. So in a way I know the forces involved in that accident can never happen on a bike. I know the limit for my body, I know I can heal. Which I guess makes my sport a bit easier now.

"I definitely got that sense of you're not invincible ... humans are fragile. And I thrive on adversity, it lights a fire in me."

Buchanan has faced adversity before. Her family home was destroyed in the 2003 Canberra bushfires.

She was a gold-medal contender at the London and Rio Olympic Games, but missed out on a medal at both events. She already has her sights on making amends at Tokyo in 2020.

"I know that if I win another world title, it's going to mean more than anything that I've ever achieved."

Her start to 2018, however, will be delayed by up to six months and will likely mean she misses the BMX world championships for just the second time in her career.

Her lungs and heart have healed, but her broken sternum is still mending and may take another four months until it's strong.

The silver-lining to the injury-enforced break - the longest of her career - is that she has time to plan a wedding with the man who helped save her life.

Adrenaline junkie Nobles has grown up on BMX ramps, motorbikes and had plenty of experience taking "dudes to hospital" before the harrowing drive with his fiancee.

"Getting injured has been a part of my life. Normally it's me in hospital. I heard my name when Caroline was coming towards me and I turned around and she had blood going down her face," a relaxed Nobles said.

"The bottom of her nose was slit and peeled back down to her lip, her chest was hurting ... it didn't really matter what was going on, we were hauling arse to the hospital anyway.

"I'm a glass half-full person. I knew if I freaked out, it would make it worse for her. Even if her face was hanging off and she doesn't realise, I wouldn't let her know. Just keep it calm and move forward."

Buchanan adds: "When you love someone, you can be a nervous wreck or take charge. Every hour Barry was checking the machines and monitors. He was unbelievable."

Nobles, who slept at the hospital with Buchanan but has returned to the US to train, will have Buchanan by his side as a "watergirl and fan girl" for a Crankworx race in New Zealand next month.

"We're fortunate that everything is OK, it could have been so much worse. But she's healthy, she's walking ... she's normal Caroline," Nobles said.

Buchanan hasn't set a return date. She has been working with AIS medical, rehab and gym staff, including Julian Jones, Greg Lovell and David Hughes, to help her recover.

She was touched by the the way fans and Cycling Australia, the sports commission and other athletes reached out to see if she was OK.

Her parents have been driving her to rehabilitation appointments and been looking after her at home. She says all of those things and her injury will make it so much sweeter when she returns.

"First of all I've just got to rebuild my body. I had an eerie feeling on the day that something was going to go wrong. For something to go wrong, it was probably the worst place for it," Buchanan said.

"But the adversity of everything now excites me, this is my biggest challenge yet."