The two worst affected victims, Joseph Ratana and Warren Burgess, were in critical condition and had to be ventilated before being flown to Wellington Hospital on the night of incident.
Workers at Tasman Tanning found them unconscious on the mezzanine floor of the tannery around 4.50pm and in a rescue bid a third worker, Taniela Balivou, passed out on the stairs. Two employees, using gas masks, were able to pull Mr Ratana and Mr Burgess to safety before emergency services arrived.
A fourth worker involved in the rescue passed out briefly.
Mr Ratana suffered the worst injuries from the gas, and was placed in a medically-induced coma for 3-4 days. He had burning and pitting to his lungs, suffered renal failure and temporary loss of eyesight and short term memory.
The court heard he was still suffering from respiratory problems and fatigue.
Mr Burgess was also placed in a coma and had burnt lungs, and temporary loss of eyesight and short term memory . Mr Balivou had ongoing eyesight problems.
Judge Gittos said the production of the gas was "immediate and overwhelming" and staff suffered injuries which were still unresolved, as the long-term effect of hydrogen sulphide was not well known.
Exposure to the gas in high concentrations could bring about severe respiratory tract irritation, acute eye irritation and conjunctivitis, convulsions, comas, and death.
Judge Gittos said it was important the chemical mixing system proceed in an "orderly fashion" because chemicals needed to be sequenced in a certain way to avoid a reaction such as the one on November 2.
"These two substances were not intended to be added at the same time."
Judge Gittos said a "human error" saw the dose of acid put into the wrong vessel, followed 20 minutes later by 97 kilograms of sodium hydrosulphide.
While the smell, similar to rotten eggs, could be detected at low concentrations, the gas could not be smelled at high concentrations when it was most dangerous because of rapid temporary paralysis of nerves in the nose, bringing about a loss of the sense of smell.
Prosecutor Greg La Hood said had the company provided employees with personal gas detectors, Mr Burgess would have been alerted to the presence of the gas when going to assist Mr Ratana.
Defence lawyer Paul Chisnall said the company trialled the gas detectors back in 2009 but found gas levels were never high enough to cause alarm so they were deemed unnecessary.It had provided the detectors following the incident.
Mr La Hood said Tasman Tanning should have had the storage tanks separate, and a proper emergency procedure in place, as other employees involved in the rescue had endangered themselves.
Mr Chisnall said the company had now taken extra measures to improve safety, including making gas masks more readily available, as workers had had to search for masks and share them.
Judge Gittos noted the difficulty in deciding reparations for the types of injuries sustained by the workers.
"It behoves the court to be not ungenerous in compensation for these men."
He ordered reparation of $35,000 to Mr Ratana and $25,000 to Mr Burgess, and the further $30,000 to the other four workers.
He said the fact Tasman Tanning fully cooperated with the investigation, provided gas detectors, and had already paid immediate costs and some reparation to the victims were mitigating factors allowing him to bring the initial fine of $130,000 down to $73,100, which he rounded to $73,000.
He said Tasman Tanning, which had been operating for 61 years and employed about 250 people, was a good employer and showed "significant remorse".
