Two workers were knocked unconscious after a gas leak at a tannery factory in Wanganui this afternoon.
A hydrogen sulphide leak was reported at the Tasman Tanning Factory on Tod Street just before 5pm.
The two men, in their early 50s, were rushed to Wanganui Hospital along with 19 others, including several fire and ambulance staff, after the spill.
Seventeen of them were later discharged but two will remain in the hospital overnight for observation.
The two workers who were overcome by the fumes have been flown to Wellington Hospital in a serious but stable condition.
The Manawatu area fire service assistant commander, Rodger Calder, said his team went in to the factory in protective sealed suits to make sure the leak had stopped, but it was too soon to say what had caused it.
Locals earlier told ONE News they "smelt a bad odour" in the air near the factory.
The tannery has been producing leather products in Wanganui since 1953. It employs about 200 people and is critical to the local economy.
One worker who ended his shift just before the accident said safety had always been paramount at the factory.
But a factory foreman told ONE News there have been some minor incidents in the past.
Government safety officials have begun an investigation.