Two tannery workers critically injured after a gas leak at a Whanganui factory yesterday are in a serious but stable condition in Wellington Hospital.

Both men have been described as ''very experienced'' senior employees with more than 20 years' experience at the factory.

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Fourteen other workers hospitalised in Whanganui last night with gas exposure have returned to work this morning.

The two worst affected men, aged 51 and 52, were flown to Wellington Hospital late last night and were being treated this morning in the hospital's intensive care unit, Capital and Coast District Health Board spokesman Lindsay Davis said.

Hunter Tait, managing director of Tasman Tanning in Castlecliff, where the gas leak occurred, said the two experienced men had been exposed to hydrogen sulphide during the tanning process.

''They are our process operators. I'd say both of these [men] have been here for in excess of 20 years. They are very experienced senior staff members, looking after the process controls of our operation.''

He said the factory re-opened this morning and all affected staff were back on duty.

''The staff are back at work. It [the leak] is being investigated. The area with the hydrogen sulphide is not yet open ... [but] part of the plant is still operating.''

Staff remain worried about the health of the injured men, he said.

''Obviously management and all staff are extremely concerned for their colleagues. They are long-term, very good employees.''

Hydrogen sulphide was a byproduct of chemicals used in the process of removing hair from the hydes. It can severely affect the lungs and breathing.

Manawatu assistant area fire commander Rodger Calder said the fire service responded to a gas leak at the Tasman Tanning factory just before 5pm yesterday.

Emergency services picked up the two men unconscious, who had been overcome by gas fumes.

Nineteen people were treated, including 14 other workers, one fireman and three St John Ambulance staff. They were decontaminated on site and transported to hospital for observation.

The leak was a big one, Calder said: "It depends on how much gas comes off and a lot of gas came off in this case. This is very serious and we have now got people in critical condition."

Two hazmat (hazardous materials) fire engines were among many fire and police vehicles still at the scene late last night.

A decontamination tent was next to an ambulance and fire fighters continued to pass in and out of the factory to ensure the leak had been sealed.

Linda Dowdle watched the scene unfold through the window of Castlecliff Bar and Grill, opposite the tannery where her husband works.

He was on his day off but she said the couple had grave fears for a fellow factory worker who is in hospital and believed to be among those in a serious condition.

Dowdle said bar patrons grew concerned when ambulances began racing past the window of the bar opposite the Tod St factory.

"I actually went out to have a look and see what was going on and the fire rescue turned up and it all started happening."

She was relieved her husband was not among the injured workers.

Tasman Tanning produces leather products and says on its website that it is the only finished leather tannery now in New Zealand.

It has more than 200 staff and processes and sells leather for upholstery, footwear, accessories and the aviation and marine industries worldwide. 

- © Fairfax NZ News