The defending Bathurst 1000 champion blamed sun glare for throwing him off his racing line before his Ford pinballed off walls approaching Forrest's Elbow at up to 170kph.
Seven marshals had to leap for safety before it slammed into their track-side enclosure.
"My biggest fear was for the safety of the marshals but to hear none were seriously injured is a big relief," Mostert said in a team statement.
"I'm really sorry for the team as it was my error that put us out and also destroyed a car.
"I just lost my racing line due to a bit of sun glare and glanced the wall.
"From there I was a passenger and took some pretty big hits."
Three of the officials were injured.
"It definitely reminds you how dangerous this joint is," Winterbottom told the V8 Supercars website.
"It's just a reality check really. It's about the big picture."
However Winterbottom wasn't so sombre after receiving a message from Mostert in his Orange hospital bed.
"One of the things he said just as they souped him up with morphine was sorry to the guys that he crashed and make sure Frosty (Winterbottom) gets pole," Winterbottom said.
"That's his true character."
Former Bathurst champ and V8 series leader Winterbottom said he was now unfazed by the prospect of launching another raid on the mountain despite his friend's crash.
"It's what Bathurst is," he said.
"You don't want to see your friends hurt ... but that's the big picture - it hits home how brutal it is."
Prodrive Racing have won the last two Bathurst titles - Winterbottom's maiden 2013 triumph and Mostert's last-to-first effort last year.
Yet Mostert has not had much qualifying luck for the team on the mountain.
He crashed at practice and missed qualifying on Bathurst 1000 debut in 2013.
And Mostert did not take part in last year's qualifying session after being penalised for passing under red flags.
He started the 2014 Great Race from the back of the grid but bounced back to seal his incredible win with co-driver Paul Morris.
"He's just destined not to qualify for this race," Prodrive Racing team principal Tim Edwards told Speedcafe website.
"(But) he's in good spirits. His comment to me was good job we signed that contract a couple of months ago."
Mostert re-signed with Prodrive Racing in August until the end of 2019.
Mostert had hoped to make a move for the championship at Bathurst, which offers double points.
He was sitting second in the V8 championship standings, less than 200 points behind Winterbottom.
- AAP
