However, Affco general Manager Nigel Stevens, who was in meetings today, said earlier the company was unable to comment until a full investigation had been completed.
E tū Northland organiser, Annie Tothill said workers at the plant learned yesterday about the latest incident on Monday night.
"We understand coal accumulated in front of the boiler door," Tothill said.
"Usually someone would open the door to inspect the boiler. But the earlier incident meant the boilerman was hypersensitive and instead shut the boiler down.
"I have been told the problem lies with the feeders that feed the boiler with coal. This system is not alarmed so problems are undetectable until pressure reaches a dangerous level.
"This is serious, and our members fear another appalling accident."
She said AFFCO told the union the boiler has been checked by an independent professional, "but it appears the assessment failed to spot the feeder was faulty."
Tothill said despite the fear of another incident, staff are too intimidated to issue a PIN (Provisional Improvement Notice), asking the company to fix the boiler.
"There is a culture of fear in this workplace and people are too afraid to raise issues for fear of intimidation or losing their jobs," Tothill said.
"Our members have zero confidence in the plant management and it's time WorkSafe took the matter seriously. It needs to act swiftly to ensure the boiler is shut down until both incidents are thoroughly investigated and the causes are fully addressed."
The union has written to AFFCO demanding itremedy the fault and undertake sustained testing and monitoring to ensure the boiler is safe before it is used again.
The union said it had also written to WorkSafe, requesting an urgent assessment by an inspector as well as an assessment of the boiler by a qualified external engineer.