Mr Williams, 27, the son of well-known local doctor Bruce Williams and his wife Sue, died in Canberra on Tuesday morning during a bunch training ride, the South Coast Register reported.

An ambulance crew attended to the cyclist on the scene on Gungahlin Drive near Belconnen Way just after 6am.

He was then transported to Calvary Hospital where he was pronounced dead.

An autopsy revealed the star cyclist suffered sudden fatal cardiac arrhythmia (an irregular heartbeat or abnormal heart rhythm).

Mr Williams, the 2007 Goulburn to Sydney winner, was a stalwart of the domestic cycling scene in the mid 2000s and a multiple champion of the Nowra Velo Club.

Starring as a junior triathlete, he won the Shoalhaven Sportstar of the Year award in 2000-2001 but made his name riding for the Drapac Cycling squad, in which he flourished as an all-rounder and included a win in Australia’s second longest race, the iconic Goulburn to Sydney.

Williams rode with the DFL-Cyclingnews.com-Litespeed team in Belgium before joining the Melbourne-based Drapac Porsche squad in 2007.

His father Dr Williams said the sudden fatal cardiac arrhythmia was impossible to foresee.

“We are just devastated,” he said.

“Some people are lucky they get warnings and can have things investigated, unfortunately for Robbie his first attack was fatal.

“The heart stops beating normally and therefore stops pumping blood.”

While unsure of what his son would have actually felt Dr Williams said they have been told Robbie had just pulled a turn on the front of the training group ride.

“Apparently he pulled off a little bit earlier than normal and went to the back of the group,” he said.

“Whether he felt funny or not, perhaps dizzy, short of breath, we’ll never know.

“I would say he would have been unconscious on the bike and dead before he fell off.

“The group said and all they heard was a crash - there was no car involved, no obstacles, no black ice - they just heard a crash.”

Mr Williams said his son was wearing a helmet at the time of the accident.

“It is something totally unforseen and unpredictable.”

It is a condition that has struck down two elite level footballers in recent months.

Italian Piermario Morosini succumbed to it in April while weeks earlier Fabrice Muamba survived a similar episode in London while playing in the English Premier League.

“They are fit and well, it is an undiagnosable condition,” Dr Williams said.

“At least [Robbie] died doing what he enjoyed. Certainly, we would all like to go that way.”

He dismissed reports that his son had a pre-existing medical condition.

“He was a fit as a Mallee bull and always was,” he said.

“While he had not competitively raced since 2010, he always maintained his fitness and was riding regularly.”

Mr Williams had not long become engaged to his fiancee Claerewyn and the pair planned to be married on New Year’s Eve.

The couple were living in Yass, with Mr Williams working in Canberra for RBS Morgan after having completed a commerce degree at the University of Wollongong.

There was talk of him possibly relocating back to Nowra to further his financial career.

A service to celebrate his life will be staged this Saturday from noon at his grandparents’ property at 226B McMahons Road, North Nowra (right at the end of McMahons Road) to which all are welcome to attend.

The family plans to hold a private service later in the week.