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Police have blasted the "brainless" actions of people throwing rocks at vehicles following a third serious attack yesterday.

Acting Superintendent John Kelly said it was only a matter of time before another person was killed.

A rock thrown from an overbridge on to the Southern Motorway at 2.15pm yesterday smashed the windscreen of a south-bound vehicle.

The badly shaken driver left the motorway at Manukau but did not notify police for 15 minutes. He is yet to make an official complaint.

The rock-thrower was described as a male wearing a black-hooded jumper. He was seen running off towards Otara.

The attack follows the death of Chris Currie on the same motorway last Friday. He was killed instantly when a concrete slab was hurled from the Princes St overbridge at Otahuhu.

A 14-year-old boy has been charged with murder.

On Wednesday, student Jenna Taylor was knocked unconscious when a person threw a rock at a bus she was travelling on in Mt Albert.

Mr Kelly said the last two cases appeared to be copycat acts.

"It's an absolutely brainless thing to do. It defies belief that anyone could think that was clever or smart."