A group of teenagers caused mayhem when they smashed windows on a train on the Southern Line in South Auckland last night.
The train's passengers had to be dropped off at Puhinui Station because the train was "munted", Public Transport Users Association spokesman Jon Reeves said.
Auckland Transport called police about 10pm.
A police spokesman said they were called to a disorder incident in Manurewa East involving a group of male and female teenagers.
"Police were told people had thrown rocks at a train and had then run off," he said.
"Police were told the group had been ejected from the train, and then proceeded to throw rocks at the train.
"The train stopped at Te Mahia station for the crew to check for damage."
A police unit attended and conducted area patrols, he said, but no offenders were located.
"Auckland Transport will assess any CCTV footage and liaise with police if offenders can be identified."
Reeves said 18 to 20 youths got on the train at Papakura and allegedly assaulted the Maori warden and train manager.
The manager stopped the train at Te Mahia Station and kicked the youths off.
They then picked up rocks and hurled them at the windscreen, shattering it in three places, Reeves said.
The train engineer then dropped passengers off at Puhinui Station.
"It sounds like this group have thought 'let's get on the train and cause mayhem'," Reeves said.