Ms Frazer and the tow-truck driver who came to her aid, Geoff Clark, both died when a passing courier truck struck them alongside the Hume Highway yesterday.

Her car had broken down on a narrow stretch of the highway, about two kilometres from the southern highlands town of Mittagong, and her car could fit only partially in the road's shoulder.

It is believed the truck driver may simply have seen the two vehicles too late.

Ms Frazer, from Springwood in the Blue Mountains and a former student at St Columba's High School, was today being mourned by her family, friends and the whole school community.

On Facebook, a relative said it was a "very sad day" because his "beautiful, intelligent cousin Sarah Frazer" had died "on her way to start a new life" at university.

Police are investigating the crash and the truck driver is assisting with inquiries.

The driver was treated for shock at Bowral Hospital.

Mr Clark, a father of four and well-known in Mittagong, was hailed as a good Samaritan yesterday for stopping on the dangerous strip of the highway and volunteering to drive the woman to Wagga Wagga.

Inspector Mark Wall from Bowral police, who knew Mr Clark, said he was "a hard worker and a good bloke".

He had been a truck driver for most of his life before starting his own towing business eight years ago.

"He was always very careful," said Steve, an employee of Mr Clark's towing business, Highland Towing, who did not want his surname published.

"He used to tell me, 'Don't take your eyes off the freeway.' I just don't know what could have happened out there."

Mr Clark had four sons aged between eight and 14 and a wife, Samantha.