It was a day like any other day for Ben Pascall and Lyndon Quinlivan. The two men went to work, as they usually would, at the Norske Skog mill near Albury in New South Wales.

But they didn't come home that night.

The two co-workers, both locals of the area, were carrying out routine maintenance at the plant on Thursday when they were overcome by fumes, thought to be deadly hydrogen sulphide gas.

Mr Pascall, 28, was found unconscious inside the mill. He later died in hospital.

His co-worker, Lyndon Quinlivan, 36 and a young father, died in the Albury base hospital overnight.

The family of a third victim, a 22-year-old North East man, have been at the man's bedside since his accident.

They say his condition has improved but: "there is still a long road ahead".

The mill is a major local employer. ( ABC Goulburn Murray: Allison Jess )

But the question remains: how did this happen?

Investigators from WorkSafe Hazardous Chemical Team are onsite over this weekend, and investigations are continuing.

The ABC understands workers from the mill have been told not to talk to media.

What is hydrogen sulphide? Known as "rotten egg gas" for its unpleasant odour, hydrogen sulphide (H2S) is colourless, flammable and potentially lethal

Known as "rotten egg gas" for its unpleasant odour, hydrogen sulphide (H2S) is colourless, flammable and potentially lethal The gas is created in the human body during food digestion, and is a by-product of some industrial and manufacturing processes

The gas is created in the human body during food digestion, and is a by-product of some industrial and manufacturing processes At low levels, the gas can cause headaches, eye and skin irritation, shortness of breath and nausea

At low levels, the gas can cause headaches, eye and skin irritation, shortness of breath and nausea At high levels, it can overcome a person and cause loss of consciousness

At high levels, it can overcome a person and cause loss of consciousness There have been suggestions the gas could be used by terrorists, and last year a group in Sydney was accused an attack with the gas

Australian Workers Union organiser for north-east Victoria and the border David Corben said yesterday the mill was a "relatively safe" site and one which they have had very minimal trouble with.

"The management were really good, and they will do their own investigation."

He said the town was in shock.

"There's a lot of tears. People are sad.

"They were both great people, I believe one of them has a couple of kids so that's very emotional. One death is too many is the feeling".

About 150 people were evacuated from the building after the leak, and 18 were taken to hospital, but 14 have since been discharged.

The site is expected to remain closed until Tuesday at the earliest.

A decision whether to re-open will be made on Monday evening. Meanwhile, workers have been stood down on full pay.