A mother has died in a hit-and-run crash at Lower Inman Valley near Victor Harbor, south of Adelaide, while riding with her husband and another cyclist.

The 48-year-old from Beulah Park in Adelaide's east was in the middle of the group, which had spread out as it moved along Inman Valley Road, when she was hit yesterday.

Police were called about 10:45am ACDT as locals tried to resuscitate her but she died at the scene.

Detective Inspector Peter Duance says another cyclist was able to describe the vehicle and two people were later pulled over in connection with the incident.

"The cyclist who was not the husband ... gave police a brief description of the vehicle involved," he said.

"One hour after this collision, police located a dark coloured [Commodore sedan] on the Southern Expressway at Lonsdale.

"An astute police officer noticed the damage to the vehicle and as a result stopped the vehicle."

The driver, a 37-year-old woman from Queensland, and the passenger, a 45-year-old man from Collinswood, were taken to the Christies Beach Police Station for questioning.

Police have charged the woman with dangerous driving causing death and leaving the scene of an accident.

She will appear in court on Monday.

Detective Inspector Duance says the victim's children have been notified of her death.

Residents rushed to help, performed CPR

Local Steven Brook says he rang emergency services after his wife Lisa, a nurse, rushed inside before going back out to help.

"Coming outside with a coffee and a smoke I saw two people standing on the bank," she said.

"I came running out here and started CPR with another RN.

"We've seen cyclists up and down here a lot and we've seen cars flying up and down here very fast so it was just an accident waiting to happen."

Investigators have appealed to any witnesses to come forward.

"It's a wide open road in the country, good visibility," Detective Inspector Duance said.

"I just urge all motorists to drive with more care when approaching cyclists in particular."

Police closed off Inman Valley Road between Swains Crossing Road and Black Valley Road as they investigated.

The death is the first on South Australia's roads over the Christmas holiday period.