A rock hurled from a motorway overpass has smashed through a taxi windscreen, glancing a driver's shoulder and narrowly averting a major tragedy.

Auckland Co-op driver, Balkar Singh has been left in shock and his vehicle is off the road after a rock struck his vehicle on the motorway as he drove a top company executive from the airport yesterday.

It's the latest in a string of disturbing incidents over the past few days in which vehicles have been pelted by rocks from a footbridge that crosses State Highway 20A in Mangere.

Co-op taxi director Harpreet Bhullar said the taxi was forced to take evasive action when a big stone dropped from the bridge and suddenly blasted through his windscreen.

Auckland Co-op driver, Balkar Singh.

He said a passenger, a top KiwiRail executive, was sitting in the front alongside the driver when the rock came through the window.

"The big stone landed inside the vehicle after smashing the windscreen hitting the driver on right shoulder while he was driving at 85km/h on the motorway towards the city," he said.

Harpreet said the police were called but unable to find those responsible for hurling the rock into the taxi.

Singh had been left nursing an injured shoulder and was now forced off the road for the next two days while his car was repaired.

"He was quite shocked. The rock hit his shoulder on the right side."

He urged motorists driving to and from the airport to be on the lookout going underneath motorway overpasses in the Mangere area.

It's the second car windscreen in the past week to be smashed by rocks thrown from the Bader Dr overbridge.

Police said a car with four people inside was hit by a rock last Saturday afternoon.

Witnesses spotted a group of youths on the overbridge at the time.

Luckily no one was injured but police issued a warning after reports of people seen throwing rocks and other items from motorway overbridges.

Counties Manukau West area commander Inspector Naila Hassan said motorists deserved to feel safe on the roads.

"This reckless behaviour from those involved is extremely dangerous and can potentially have fatal consequences," Hassan said.

She said people needed to remember that throwing anything from overbridges posed a serious risk to vehicles and their occupants.

In 2005 apprentice builder Chris Currie was killed after he was struck by a 8kg piece of concrete thrown from a bridge on the Southern Motorway.