A toxic fume from a blast at the Mount Arthur mine in the Upper Hunter turned the sky bright orange and prompted demands for a much stronger response from environmental regulators.
Ammonium nitrate and fuel oil were detonated at the mine near Muswellbrook on Wednesday afternoon, causing poisonous fumes containing nitrogen dioxide to spread several kilometres from the site.
Workers in the Muswellbrook industrial area said there were no warnings about the toxic cloud, which left people with sore throats.
The blast led to an apology from BHP Billiton.
Mine workers were told yesterday that the ammonium nitrate and fuel oil were in the ground for 21 days, seven days longer than the recommended time in which blast material should be detonated.
The Environment Protection Authority is investigating the blast, including the length of time the materials were left in the ground.
Mine workers told the Newcastle Herald that blast material, boosters and detonators are left to "sleep" in the ground for longer than the recommended 14 days because of weather conditions.
Blasts can also be delayed for production reasons.
A blast technician from another mine told the Herald that ammonium nitrate and fuel oil, the most commonly used blast materials in the Hunter, were highly susceptible to water contamination.
The dark orange colour of the fume meant the whole "shot" was likely to have been contaminated, he said.
Muswellbrook mayor Martin Rush said the community would find the incident disturbing.
Cr Rush and NSW Nature Conservation Council chief executive Pepe Clarke said the botched blast, less than three months after Mount Arthur was fined for blasting incidents in October last year, showed the company had a "serious culture problem" and was "cutting corners" and risking people's health.
"Given a risk assessment requires a consideration of both likelihood and consequence, and given the probability of a wind change and the heavy consequences arising from a population exposed to fumes, the risk must have been considered to be high," Cr Rush said.
"The fact that the blast proceeded will leave people with the impression the company put profit before people.
"The community will also find it disturbing that the company has offered up excuses in circumstances where zero tolerance conditions were operating, supposedly to ensure health and safety."
Muswellbrook Council could seek independent monitoring of each blast at the mine site.
"It may be necessary for council to ask the Planning Assessment Commission to consider imposing conditions requiring an independent assessment of each blast if BHP can't ensure the community's safety," he said.
Mr Clarke said it was a serious concern that a large mine such as Mount Arthur was managing its blast activities so poorly.
"It's clear the $1500 fine imposed less than three months ago is completely inadequate to deter blasts of this kind and irresponsible conduct by this company," he said.
"The company knows perfectly well how to reduce their polluting blast incidents but they continue to cut corners.
"Exposure to high levels of nitrogen dioxide blast gases can cause severe harm."
Of particular concern was the lack of research available about pollutants within blast fumes, Mr Clarke said.
The Queensland Department of Natural Resources and Mines notes that "any atmosphere in which nitrogen dioxide is noticeable by smell, irritation and colour should be regarded as potentially dangerous".
The department's website notes a 2006 Hunter Valley study was the first in the world to study pollutants within blast fumes.
An 18-month Queensland study is investigating fumes.
EPA north branch director Gary Davey said it could take "regulatory action" against the company.
Mount Arthur Coal issued an apology, saying it took its environmental and community obligations very seriously.
NSW Energy Coal Asset president Peter Sharpe said the blast was timed to ensure that wind conditions would prevent any fumes from drifting off site.
"However, due to a change in conditions immediately after the blast, some fumes did travel towards the industrial area," he said.