A veteran taxi driver says he is lucky to be alive after a sharp rock blasted like a projectile through his windscreen striking his body as he drove along an Auckland motorway.

Balkar Singh is recovering after yesterday's close call left him nursing an injured shoulder. His taxi is off the road while the shattered screen is repaired.

It's the second vehicle in less than a week to have been damaged by rocks hurled from overbridges in Mangere.

In yesterday's terrifying incident the Auckland Co-op taxi driver said he sensed something was amiss when he spotted a group of boys standing on an overbridge over the Southwestern Motorway airport link yesterday afternoon.

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"I was thinking they are going to drop something because they are holding something so I just tried to avoid them but I couldn't.

"Soon a big stone landed on my windscreen and glanced off my shoulder.

"I was lucky to be alive."

He said the moment the sharp-edged rock - which weighed at least 3kg - hit, the windscreen exploded.

"I had glass all round my face and all around my shirt and even on my customer because he was sitting in the front seat with me.

"Luckily I didn't lose control of my car so I quickly moved on to the left and stopped in a safe place and then we called the police."

He said it was a terrifying and horrible ordeal which had left him in shock.

He was thankful the rock struck him on his shoulder missing his head by a fraction.

"I'm still having pain in my shoulder."

Police were today stepping up patrols in the area and vigilance around motorway overbridges following the spate of "reckless" stone throwing.

Counties Manukau Police district command centre manager Inspector Dave Simpson said police were called yesterday to reports of youths throwing stones off the Bader Dr overbridge and off the footbridge by Montgomery and Kirkbride Rd.

At least one vehicle was reported to be have been struck by a large stone.

Multiple police cars were sent to the area but those responsible had disappeared.

Simpson said while police attempted to find the youths a number of young people in the area were spoken to and warned against the dangers of taking part in this reckless activity.

"Police are taking this matter seriously and warn those involved that their actions will not be tolerated," said Simpson.

On Saturday a car with four people inside was hit by a rock thrown from the Bader Dr overbridge.

Police issued a stern warning at the time saying the reckless behaviour was extremely dangerous and motorists deserved to feel safe on the roads,

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.

The Road Transport Forum said truck drivers were becoming "increasingly nervous" about using urban motorways and action was needed to stop rocks being hurled from overbridges.

Forum chief executive Ken Shirley said drivers had reported an increase in the number of rock throwing incidents with two trucks from the same company suffering shattered windscreens at Mangatawhiri and on the Kapiti Expressway.

"Understandably drivers are becoming increasingly nervous about taking these routes at night. Authorities must take a pretty hard line on this kind of delinquency," said Shirley.

"While nobody wants to see cages around motorway overbridges all options must be considered," says Shirley. "It is only a matter of time until somebody else is killed."

Simpson urged anyone who witnessed people dropping rocks on to vehicles to call 111.