Auckland Transport is warning that someone will be seriously injured or killed if young people continue to graffiti trains.
The warning comes after an incident Wednesday.
CCTV footage shows a group of youths wearing balaclavas spraying paint on a train as it slowed and stopped before entering the Parnell tunnel.
Auckland Transport, Manager Rail Services, Craig Inger says being anywhere near train tracks, at any time, is stupid and dangerous.
"It's also costly, with ratepayers and taxpayers forking out tens of thousands of dollars a year to cover up the damage.