A passenger says people wearing masks spray-painted an Auckland train last night, filling a packed carriage with fumes.
The incident happened on the Waitakere line just before 7pm.
A passenger said the fumes filled the carriage of the train when it stopped unexpectedly between Britomart and Newmarket stations.
She said the smell and fumes were overwhelming, some passengers were distressed and an elderly woman told her she was struggling to breathe.
The woman said passengers banged on the windows at the taggers but they kept painting for about 15 minutes.
Auckland Transport said it would review CCTV footage of the incident.
A spokesperson for the agency, Mark Hannan, said it appeared people were just being stupid but it was still taking the incident seriously and would be looking to tighten security.
He said the agency would speak to witnesses today and would hand over the footage to the police.