A tagged train on the Johnsonville line. A witness said people with covered faces filmed themselves spraying the train for at least five minutes.

A group of masked people brought a train to a halt near Wellington before vandalising it and filming the event.

A KiwiRail spokeswoman said police were investigating an incident on the Johnsonville Line on Friday.

A passenger, Scott Hayles, said he was travelling on the 1.02pm train from Wellington to Johnsonville when five or six people spray-painted the train.

After the train had stopped at Awarua Street station, near Ngaio, one or two of the group members boarded, while the others remained hidden off the train. They sprung into action when the train started moving again, Hayles said.

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"One of them grabbed the emergency brake [to re-open the doors]... They jumped out... they looked pretty organised."

As the masked group started vandalising the train, the driver and guard appeared powerless to stop them.

"The guard jumped out and yelled... he wasn't going to have a go," Hayles said.

He said the group did not seem aggressive or intimidating.

"They sprayed the train for at least five minutes, and they filmed themselves doing it."

The group eventually ran off, as the train manager called police and relayed events over the phone.

Hayles understood the incident he witnessed was one of two similar events on Good Friday.

A KiwiRail spokeswoman said police were investigating.

"When it happened this morning the train manager rang the police immediately."

The incident on the publicly-owned train followed a similar March 14 event in Melbourne, when vandals forced a train to stop before graffiti-bombing it in front of passengers.

The NZ Railways website said FP5246, the train vandalised, was built in 2015, making it one of the newest Matangi-class trains.