Waste Management has been fined for the tragic death of a worker who was chemically poisoned at its treatment pit in Lower Hutt.

Jim Gideon died in August 2017 after being overcome by hydrogen sulphide gas from a pit at the Seaview facility.

Gideon had been told to carry out a dangerous mixing of reactive chemicals to treat hazardous waste from the Haywards substation, WorkSafe said.

The waste had sat unmarked at the site for six months before being dealt with. During this time, the waste was never officially recorded, nor tested or labelled by the site chemist. A destruction certificate was issued despite the waste having not actually been destroyed.

On the day of the fatal poisoning, the hydrogen sulphide alarm repeatedly rang at the site, but work continued regardless.

Gideon collapsed mid-afternoon, after exposure to at least 500 parts per million of hydrogen sulphide. The maximum workplace exposure allowed is 10 parts per million over an eight-hour period.

WorkSafe's investigation uncovered a breakdown of systems at the site, with health and safety failures at every level. These included improper storage and hazard identification, a lack of personal protective equipment for workers, and inadequate risk assessment.

"Jim Gideon's death was completely unnecessary and avoidable. It's only by luck that there were not multiple fatalities at Seaview that day," said WorkSafe's national manager of investigations, Hayden Mander.

"Exposing workers to this degree of danger is unacceptable to WorkSafe, and negligent in the extreme. Waste Management's failure on so many levels is appalling."

In sentencing at the Wellington District Court yesterday, Judge Bruce Davidson said a "wholesale systemic failure" had led to the creation of a "fatal gas chamber" at the facility. He imposed a fine of $450,000 and ordered reparations of $360,000.

Waste Management NZ Limited was sentenced on charges of exposing an individual to risk of death or serious injury or illness. The maximum penalty is a fine up to $1.5 million.

In July last year, Waste Management's managing director Evan Maehl wrote an email to staff about the "tragic and avoidable death".

"Jim had been a valued Waste Management employee for nine years," he said.

"As we have previously expressed to the people most closely affected, we were shocked by his death. We deeply regret the loss of Jim and unreservedly apologise for the pain caused to his family and loved ones. Our thoughts continue to be with them."

He said Waste Management had pleaded guilty to failing to comply with health and safety duties, but defended a charge of recklessness on the basis they would never knowingly place a worker in harm's way. They were found not guilty on that charge.

"As some of you may be aware, we have since had all our operating procedures and systems independently reviewed and a detailed and wide-ranging corrective plan was implemented in full by 2018.

"This included a new safety approach with an increased focus on process safety, management of change and a new 6x6 risk matrix for our Technical Services division."

He wanted to reinforce the death was a "serious and tragic reminder" to follow safety systems and remain vigilant of the dangers they faced as a team.

" I, and the EMT, exhort you all to always put safety as your first priority, every day."

Waste Management has been contacted for comment following yesterday's sentencing.