According to an earlier High Court ruling on the case, the waste was misclassified as mercury water, a substance with low levels of mercury that doesn't pose risk to human health.
There were six containers of the waste, each holding 1000 litres of sodium sulphide solution and a mercury sulphide precipitate. Because it was misclassified, the relevant workers believed they were dealing with mercury water.
On the day of the fatal poisoning, the yard supervisor decided the day's work would involve processing the six containers by bringing the ph level up to nine to make the mercury fall out of the water.
But the actual substance already had a high ph level, meaning the process they used caused a reaction, releasing hydrogen sulphide gas.
A hydrogen sulphide alarm on site went off several times, and each time workers left the building to wait for the levels to drop.
"What seems to have been insufficiently focused on was that the alarm should not have
been going off so regularly if the waste was mercury water," the High Court ruling said.
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 said.
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 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.
