A worker died after being overcome with gas at the Norske Skog plant at Ettamogah.

The man who died was one of three who were taken to Albury hospital in a critical state after being left unconscious while working atop an indoor water tank on Thursday.

Norske Skog Albury mill general manager Milo Foster, along with his staff, was devastated by the death.

“It’s tragic and our condolences go to all the family and the friends of the gentleman,” Mr Foster said.

Four workers, three Norske Skog staff and a contractor, were on the tank checking valves when trouble hit between 1pm and 1.30pm.

“They were apparently all overcome, to varying degrees, by a gas, we think it’s hydrogen sulphide,” Mr Foster said.

“One of the employees issued a ‘man down’ alarm and people came to help get them out.”

They were 10-15 metres above the ground, in the paper machine building.

Ladders and ropes were used to bring the three unconscious men down from the tank.

They were taken by ambulance to hospital, with an Albury Wodonga Health spokeswoman saying one died shortly after arriving while the others remained in a critical condition on Thursday night.

A further 12 were treated for minor effects and discharged.

Mr Foster said the factory had been preparing to restart production after a three-day annual shutdown for maintenance. 

Mr Foster said hydrogen sulphide was suspected to be the gas, based on previous experience of what occurred during down periods.

“It forms when things have been sitting for awhile, from the pulp used to make the paper,” he said. 

Hydrogen sulphide is also known as rotten egg gas due to its odour. 

Firefighters ordered the mill to be evacuated after they were called to Thursday’s incident.

About 200 staff and contractors were on-site at the time.

SafeWork NSW staff descended and they will undertake an investigation, with police also at the scene.

Mr Foster said it was unclear when production, which had been due to restart on Thursday night after the shutdown, would resume.