Vancouver police have launched an investigation after the discovery of an abandoned two- to three-day-old baby at a bus stop over the weekend.

A woman waiting at a bus stop discovered the baby when she noticed a large plastic bag.

Vancouver police spokesperson Const. Anne Drennan said when the bag started moving, the woman looked inside and found the baby girl.

"It's very unusual in Vancouver to have a story like this where you have an abandoned baby where we just have absolutely no idea where the baby might have come from," she said.

The baby is being treated in hospital for an undisclosed medical condition. Police said they want to find the parents to find out the family's medical history.

"We're asking that the parents of this child come forward, as obviously they would be the most appropriate ones to provide us with background information that could help the hospital staff with the most effective treatment for the baby."