A Blue Mountains mother has been charged after she allegedly left her three-year-old daughter in the back of her car with the windows closed while she went shopping.

Police were forced to smash the rear window of the Subaru to free the girl after she was seen abandoned in the car in an underground car park at Katoomba just before 6pm on Monday.

The officers had to force their way into the car because the windows were wound up and the doors locked while the girl also did not respond to their knocking or calling out.

It is not known how long the girl had been alone in the car.

Police said they were still trying to wake the child and cool her down when her 29-year-old mother returned to the car.