A driver following in another car called emergency services.
A specialised tow-truck was at the scene with a long winch rope to drag the car back up to the road, police said.
Wellington Police constable Wendy Bryant said the mother was driving back to Johnsonville Plunket at the time to get the baby capsule re-checked.
"Obviously it was properly installed as it possibly saved baby's life."
Police said the seat belt and the properly-fitted baby capsule saved the pair from serious injury.
"Mum was understandably shaken but although both her and her son have suffered no injuries they were taken to hospital to be checked out anyway," Ms Bryant said.