A gang of as many as 12 people graffiti-bombed a train during a stop at Auckland's Baldwin Ave station in Mt Albert on Sunday afternoon, delaying passengers on board.
Video has emerged of the attack where scores of youths can be seen covering the train in spray paint while passengers were trapped inside.
The youths pulled an emergency tag on the outside of the train which stopped the train from continuing its service, allowing the gang to do its work, Auckland Transport spokesperson Mark Hannan told Newshub.
"The incident at Baldwin Ave yesterday was completely unacceptable. This is a wider issue with society, and not confined to public transport," he said.
"Thankfully it is relatively uncommon and the vast majority of people using the public transport network are law-abiding and respectful of what is public property."
Police were called to the scene however the gang had escaped by the time they arrived. Police are now investigating, including looking at CCTV footage.
The train has already been taken away and washed off.
"Our advice to passengers is, even though they are as angry as the rest of us, is not to put themselves in danger by engaging with these idiots," Mr Hannan said.
Graffiti attacks on Auckland trains aren't a new issue for Auckland Transport. Last year, it cost ratepayers around $1.2 million in clean-up and damage costs.