Auckland Transport is warning someone will be seriously injured or killed if foolish young people continue to graffiti trains.

The warning comes after an incident yesterday where youths wearing balaclavas were seen spraying paint on a train as it slowed and stopped, before entering the Parnell tunnel. Fumes from the paint overwhelmed those onboard.

The incident was captured on CCTV footage.

Rail Services manager, Craig Inger, says being anywhere near train tracks, at any time, is stupid and dangerous.

He says it's also costly, with ratepayers and taxpayers forking out tens of thousands of dollars a year to cover up the damage.