A car windscreen was smashed by rocks being thrown from the Bader Drive overbridge in Manukau last Saturday afternoon.
Police spokesman Nicholas Baker said the incident happened at around 3.40pm and witnesses spotted a group of youths on the overbridge at the time.
"A vehicle had its windscreen smashed...the four occupants were lucky to avoid any serious injuries," Baker said.
Police issued a warning following concerns over a recent trend where people had been 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.