UPDATED 9.16AM: A group of young people have caused so much damage to a train on Auckland's Southern Line, it's had to be taken out of service.

Passengers were dropped off at Puhinui Station last night, after rocks were thrown at the train and its windscreen was smashed.

A Maori warden was assaulted, when a group of about 20 young people got on the train at Papakura.

The train manager kicked them off at Te Mahia Station but an emergency stop button on the outside of the train was pushed, and this stopped the train.

The young people then picked up rocks and threw them at the train.

Public transport users association spokesman Jon Reeves said the train's windscreen shattered in three places.

"It sounds like this group have thought 'let's get on the train and cause mayhem'."

The train was travelling northbound from Papakura, heading to Britomart.

Police are investigating and CCTV footage is being used today to identify those involved.

In a separate incident on the line from Onehunga to Britomart, the emergency stop button was also pushed and the train vandalised. 

Reeves is calling for better security on the trains.

Until a proper rail security team is brought in, there should be a dedicated team of police on the network, especially at night, he said.

"It is clear it is at night when the issues are happening. There needs to be rail police. There the only ones who really have authority and can really stop things and really stop those sort of people even boarding the trains."