The Rail and Maritime Union says KiwiRail workers were gassed three times in the Otira Tunnel on the West Coast last year.
KiwiRail has confirmed one incident in November last year involving seven maintenance workers.
The 8km tunnel had been closed to passenger trains from July to November because it posed a fire risk and extra maintenance was needed.
KiwiRail said an alarm sounded warning workers of gas as they were leaving the tunnel on 6 November.
But the Rail and Maritime Union says workers were gassed on three occasions by a build-up of exhaust fumes from vehicles parked inside the tunnel.
It says the workers suffered nausea and their health needed monitoring, but they are fine now.
KiwiRail and Worksafe New Zealand are investigating.
