South Australia Police said a 32-year-old woman suffered serious injuries and died in hospital in Adelaide overnight.
A statement added that a man, 33, had been arrested and charged with causing death by dangerous driving, driving without due care and endangering life.
The driver was known to the woman and has been released on bail until March.
It has been widely reported in Australian media that the man and woman are Dennis and his wife Melissa Hoskins.
Hoskins was a world champion in the team pursuit in 2015 and raced at the London 2012 and Rio 2016 Olympics.
She and Dennis married in 2018 and have two children.
In February, Dennis announced he would retire at the end of the 2023 season after a career in which he won stages at the Tour de France, Giro d’Italia and Vuelta a Espana.
A multiple world champion on both road and track, he won road time trial bronze at the Tokyo 2020 Olympics, having won team pursuit silver at London 2012.
CPA Women, the worldwide association of women cyclists, said it was “shocked” by the news of Hoskins’ death, adding: “We send our thoughts to those who love her, starting with her children, and were lucky enough to ride with her during an all too short life.”
