Two workers went into cardiac arrest and a third was transported unconscious. A fourth man was seriously ill. The spokesman said the men had been "overcome by fumes".

A 28-year-old man from Wodonga died on Thursday, and another worker aged 37 from Lavington died on Thursday night. A 22-year-old man remains in a critical condition in hospital.

Norske Skog Albury mill general manager Milo Foster told reporters on Friday afternoon the company was focusing on providing counselling to employees struggling with the loss of two colleagues.

"The mood is pretty sad right now, there is a lot of concern particularly for the families of the two employees," said Mr Foster.

"One of the employees had a couple of small children. They were very tight, that particular crew."

The machines at the plant would not be restarted until investigations were completed and there was no danger to the community from the leak, he said.

About 150 employees were evacuated on Thursday night, with another 15 taken to Albury Hospital.

On Friday morning, 14 workers had been released and one more remained in hospital in a stable condition.

SafeWork NSW regional manager Scott Kidd said the mill had been doing maintenance work after a routine three-day shutdown when the incident occurred. CPR was performed before paramedics moved the injured to hospital.

"As a local I have a number of friends who work here and I was at soccer training last night and there's a couple of guys who work at the mill and they were quite upset,'' Mr Kidd said.

''There will be a lot of people in the community that know these people and it's such a tragic event."

A "long and complex" SafeWork investigation into the incident has started, with hazardous chemical specialists travelling from Sydney to assist.

The gas involved has yet to be formally established, but both a mill manager and a SafeWork investigator believe it was most likely hydrogen sulphide.

"The injuries and the related fallout from what occurred yesterday is consistent," he said.

"The effects of hydrogen sulphide can be quite tragic, but they are rare."

It is believed three Norske Skog staff and a contractor were on a tank about 10 to 15 metres above ground checking valves between 1pm and 1.30pm on Thursday when they fell ill.

“They were apparently all overcome, to varying degrees, by a gas,” Mr Foster said.

One of the employees issued a ‘man down’ alarm.

Workers rushed to help get them out.

Mr Foster said it was likely the gas may have been hydrogen sulphide, based on previous experience of what had occurred during down periods.

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

Norske Skog provides paper to publishers and commercial printers, and has another mill in southern Tasmania. The company's Albury mill produces 274,000 tonnes of paper annually and has 184 employees.

A fire tore through the mill last month, with 50 firefighters called to extinguish the blaze which engulfed about 300 tonnes of recycled paper. No-one was injured and police did not treat it as suspicious.

Local AMWU organiser David Corben said both of the deceased men were members of the union. He said he was at the site on Friday to meet with management.

"We are all on the same page to make sure the site is safe, the incident is properly investigated and to look after people's wellbeing and mental health," he said.

Counselling is being arranged for workers.

- with The Border Mail