Members of Orica's community reference group say they were kept in the dark over the exposure of Kooragang Island workers to extremely high levels of potentially deadly cobalt dust.
The company was fined $1.2 million after pleading guilty to repeatedly exposing workers to high-risk dust particles known to cause serious illness and death.
Despite knowing workers were being exposed to levels of cobalt dust - between 24 and 56 times the level which "should have rung alarm bells" - nothing was done for years.
One of the two affected workers has been diagnosed with occupational asthma.
Members of the plant's community reference group, who attend regular briefings on matters of relevance, said they had a right to know about the long-running exposure issue.
"We get reports about all sorts of incidents at every meeting but never once have we had a report about this," Correct Planning for Mayfield representative John L Hayes, who has been a reference group member since 2011 said.
District Court Judge David Russell said Orica knew that the ventilation system in the cobalt shed was not working properly as early as July, 2014 but took no steps to address it.
As part of an apology issued following the judgement Orica acknowledged its conduct "fell below what was acceptable".
The Kooragang plant has been largely incident free since an infamous hexavalent chromium leak in 2011 resulted in the chemical being sprayed over homes in Stockton.
Stockton resident and long-time reference group member Keith Criag also learned about the most recent incident via the media.
"I can only say I feel let down," he said.
"They have a safety section at each meeting and there hasn't been a mention."
Mr Criag said there were issues arising from the poor ventilation of cobalt that had potential implications for surrounding communities.
"This all came about because they weren't ventilating the cobalt properly. If they ventilate it properly, where does it go," he said.
An Orica spokeswoman said reference group members would be briefed on the judgement at the next meeting.
"As part of our planned engagement, we have notified the Orica Community Reference Group about the judgement and will further brief members at the next scheduled meeting later this month," she said.
Mr Hayes said he spoke to the plant's manufacturing centre manager Viney Kumar on Thursday to express his disappointment.
"I told him that this had been going on for years and years and there were people on the sick list. I made it clear they should have told us (the reference group) about it," he said.
The company's latest apology for the incident follows an apology issued last year following an unplanned shutdown of the ammonia plant.
The unplanned maintenance was needed to address a minor pipe leak.
However, the steam venting process resulted in a series of loud bangs that were heard across Stockton.
