A woman found passed out behind the wheel at a Melbourne supermarket had a blood-alcohol level more than six times the limit, police say.

The 24-year-old woman was found unconscious in her car by police at a supermarket car park at Seaford in Melbourne's south-east on Friday night.

The woman was believed to have been involved in an earlier crash at Frankston where she lost control of her car and hit a pylon, police said.

She then went to buy alcohol at the supermarket and passed out behind the wheel.

The woman, now in custody, allegedly blew 0.345 on an alcohol breath test.