Paramedics have been called to a shopping centre car park in Sydney's north to treat a three-year-old girl who had been left locked in a car for up to 90 minutes.

Police are investigating the incident which happened at an underground car park at Belrose.

Emergency services were called after a bystander reported finding the girl locked in the car at 1pm (AEDT).

Ambulance Service NSW spokesman Ian Johns says the child was sweaty and distressed when she was released.

She did not need to be hospitalised but he says it could have been a different story.

"Babies and small children are unable to regulate their body temperature as efficiently as adults," he said.

"The temperature inside a car can soar to over 50 degrees within a few minutes. Even in milder temperatures such as today, children and babies can get sick very rapidly.

"No matter how quick you think you are going to be, never leave a child unattended in a car. Always take your child with you."

Paramedics say anyone who finds a distressed child locked in a car should call triple-0 and stay with the child until the ambulance arrives.