A KiwiRail employee has been exposed to asbestos after working on part of a train that wasn't correctly labelled to alert staff of the mineral's presence.
Asbestos was first discovered in 40 locomotives imported from China in 2014.
Two years later, it was also found in gaskets - a seal used on the brakes.
KiwiRail was meant to put warning stickers on all contaminated locomotives.
But an employee at the Hutt Valley workshop came in contact with a gasket that later tested positive for asbestos, and had no warning sticker.
KiwiRail spokeswoman Katie McMahon said the man didn't touch the asbestos.
An audit was underway to ensure all gaskets of the same make were labelled correctly.
"We're looking into why that's happened so that we can ensure that it doesn't happen again."
The Rail and Maritime Transport Union said KiwiRail's lack of labelling on a locamotive contaminated with asbestos was a disaster.
Union spokesperson Wayne Butson said the labels were not effective.
"Putting an adhesive label on a toaster, because that's essentially what the dynamic grids are - a sticky label's not going to last.
"We assumed they would actually be permanent labels that would actually be fixed to the locamotive," he said.