Firefighters were called to help on Saturday after a mother accidentally locked her 9-month-old child in a car.

Whanganui Fire station officer Shane Dudley said the crew were called to the post shop about 11.40am to help free the baby from the car.

"The 9-month-old was sitting back, smiling away," he said.

They were able to use a coat hanger to open the car door.

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If the baby had been in distress, they would have used a faster method, Mr Dudley said.

"We chose to take our time and use a little bit more finesse."