Two young girls were alone with their dead mother in the wreckage of a car for up to nine hours after crashing into a tree at Bega on the New South Wales South Coast, police have said.

The girls, aged three months and two, were still in their child restraints when police found their overturned vehicle with the body of 33-year-old Jacqui Britton in the driver's seat on Monday evening.

Ms Britton had been driving home from the local swimming pool with her daughters when she hit a tree and rolled down an embankment.

Jacqui Britton's car hit a tree and rolled down an embankment on Tathra Road, Bega. ( ABC News: Joanna Woodburn )

Earlier in the evening, the woman's husband told police he had been unable to contact his wife since that morning and she had failed to return home with their two children that afternoon.

Family and friends joined police on a search for Ms Britton and the girls.

They backtracked the route Ms Britton usually travelled home from the swimming pool and found the overturned car on Tathra Road, just past Kerrisons Lane at about 7:30pm.

The girls suffered minor injuries and were initially taken to Bega District Hospital for observation.

The family's pet dog also survived the crash.

Police Commander Peter O'Brien said their car appeared to be the only vehicle in the accident.

"It looks like a tragic accident where the vehicle has left the road for reasons unknown at this stage and collided heavily with the tree," he said.

"It hit the tree and rolled down an embankment and as a result, it was difficult to see from the road, hence the time delay in locating the vehicle was about eight or nine hours.

Jacqui Britton's family and friends have posted moving tributes on Facebook. ( Supplied: Facebook )

"From our enquiries it looked like the car was there for approximately eight to nine hours with the children intact in their child restraints.

"With the family as well was a small pet dog in the vehicle."

Officers are now investigating the cause of the accident and a report is being prepared for the coroner.

Commander O'Brien said while both children were unscathed, the youngest would be flown to Sydney for further observation.

"Luckily two young females, two years old and three months old were found in the car," he said.

"The youngest of the two siblings is going to be airlifted to Sydney just for routine checks due to her tender age."

He said the children appeared to be in a good condition when they were found.

"The children were fine. They were a little distressed but not overly distressed. They were dehydrated slightly," he added.

Family and friends have posted moving tributes on Facebook, and Jacqui's sister Emma Britton vowed to help her children.

Jacqui Britton's overturned car was found on Tathra Road after she was reported missing by her husband. ( Supplied: Facebook )

"Jacqui Britton, you were my sister and my best friend," Emma posted.

"I don't know what we're going to do without you. I'm so lucky to have been able to share 33 years with you.

"My heart is broken.

"Everyone, thank you for all the private messages and flowers.

"We are lucky to live in Tathra and I know the community will rally around ... in these trying times.

"I will look after your family, Jac xxxxx."

Crash latest in series of tragedies near Bega

Commander O'Brien said Ms Britton's death was a tragic loss for both her family and the small town of Bega.

"The family have stuck together and have been spoken to by police and obviously it is a very tragic and sad situation for the town of Bega, but more so for the family and it is just another reminder we have to slow down on the roads," he said.

Local priest Father Michael Lim said it was the latest in a series of tragedies in the area.

He said the accident shocked the local community.

"It certainly affects them," Father Lim said.

"We have [had] a couple in the last year, so it certainly affects us tremendously and [effects] the people here in this small community."

A memorial fund has been set up for Ms Britton's family.

NSW Roads Minister Duncan Gay said he was also shocked by the accident.

"That was a terrible accident overnight and I'm sure anyone that's aware of the circumstance couldn't help but be moved by what happened to that family, those two children and a husband at home waiting for them," he said.