Marsden Point oil refinery has run prohibited training exercises using thousands of litres of firefighting foam containing chemicals linked to cancers.
Its fire crews used 60,000 litres of the foam in nine exercises as recently as a few weeks ago, despite a nationwide ban on training with it.
The refinery said today it was testing its site and Whangārei Harbour after it used the foam.
"We've undertaken testing on our site and in the harbour and further testing is ongoing.
"From testing results to date we don't believe there has been any significant discharge to the harbour and we are obtaining advice on what further remediation might be required on site."
A firefighter is understood to have blown the whistle.
Refining NZ called in a private investigator who reported back to it last week.
That report, obtained by RNZ, blames the site's fire manager, and speculates he was under stress as was about to be laid off and that he was afraid his crew was going to be left "vulnerable" if they did not train with the foam.
It is not illegal to store the foam but training with it, by anyone, was banned in April.
The Northland Regional Council said the foam's use appears to be "unlawful".
The man-made PFAS - per-and-polyfluoroalkyl substances - pose a threat at the level of a drop in an Olympic-sized pool, build up in the body and last thousands of years.
Asked yesterday about the pollution, Refining NZ said it was a "disappointing occurrence".
It had been "made aware" of it recently, but did not say how.
"This foam is currently permitted ... for use to fight fires, though it cannot be used for training purposes," it told RNZ in a statement.
First Union said the refinery had either been disorganised or negligent.
The company said it was bringing in stormwater treatment and taking other mitigating measures. All fire training had stopped.
The refinery sits on the Marsden-Ruakaka aquifer. It is understood the fire training ground is mostly lined with concrete.
The company sent RNZ an advisory it gave the sharemarket a few days ago in which it told the NZX it had reported "an environmental non-compliance that occurred on nine occasions".
The Environmental Protection Authority is investigating.
The council said it was alerted on 22 June, was still investigating, and then would decide on any enforcement action. It said Refining NZ had told them it was now testing its stormwater discharge into the harbour, shellfish, sediments, soil and groundwater
Refining NZ is on the cusp of a key shareholder vote today, on whether to turn itself into an import-only terminal.
A First Union delegate at the refinery said firecrews got covered in the foam during the training.
"It goes everywhere, it sticks to everything ... in your clothes," operators' delegate Aaron Holroyd said.
"We've just been using foam that is potentially toxic to our members, toxic to the environment, no one's told us, and we discover that after the event."
The union was seeking health and environment tests, and a lot more information about how it happened, he added.
The country has been trying to phase out the foams since the Defence Force revealed almost four years ago it had contaminated water and land around its military bases with PFAS, which comes in thousands of different types. They are in a class of toxic persistent organic pollutants of which there is worldwide agreement on getting rid of.
This comes 20 years after the US first raised the alert about the threat, exposed in Teflon, that began an avalanche of lawsuits and huge payouts that became the subject of a Hollywood movie, 'Dark Waters'.
The Refining NZ investigation says PFAS foam had to be stored to meet the minimum levels of foam it must have on hand in case of emergencies.
Some industries say these foams are better at putting out fuel fires than new, fluorine-free foams.
Asked where the foam has dispersed to, the refinery did not say. It was obtaining expert advice on potential effects and how to fix them, it said in a statement.
It refused to do an interview.
Crews trained with the foam between 14 April, and 5 June, the investigation shows.
Six hundred litres of Thunderstorm foam concentrate was mixed with 60,000 litres of water.
"It is clearly marked as Thunderstorm and labelled to state it is not to be used unless absolutely necessary," investigators said.
Staff were told another type of fluorine-free foam was meant to be used for training.
A stocktake in July 2020, next to the entry for 9000 litres of Thunderstorm concentrate - enough to make a million litres of foam - said: "Can be used as a last resource [sic]", the investigation report said.
The investigators - the Risk Management Group chaired by former police commissioner Mike Bush - made assertions about the fire manager, saying they "believed" this and that.
Refining NZ did not address RNZ's question of whether it had contacted the fire manager.
"We consider that the fire manager is responsible for the use of Thunderstorm foam during training exercises on those dates," the investigation said.
"His level of knowledge, and whether this was a deliberate decision in order to gain 'real life' experience for his team, as opposed to confusion as to which foam was being used, is unclear.
"We understand that the fire manager was at the time of the discharges going through a termination of employment process (although the reasons for that are not connected to these discharges).
"We consider that the stress of losing his job, and the fear that he was leaving his team vulnerable without having experienced staff to handle an emergency response scenario, may have been contributing factors to his decision-making process."
Staff were interviewed with a Refining NZ manager present, who took notes.
"While appropriately conducted by Refining NZ, we suggest that this investigation may have resulted in further stress and anxiety to other Refining NZ staff and we suggest this should be considered by management," the investigation said.
Most staff face being laid off if the refinery switches to import-only next year.
The regional council said: "Based on the information provided ... to date and a check of the information provided in [the refinery's] resource consent application, the discharge of the foam that it used is not expressly allowed (authorised) by Refining NZ's resource consents and therefore was unlawful."
Iwi have been approached for comment.
The EPA launched an investigation into PFAS use nationwide in late 2017, but did not prosecute the companies it found had broken restrictions imposed a decade ago.
The largest-ever medical-screening study in the US, called the C8 study, confirmed a probable link from exposure to PFAS to ulcerative colitis, thyroid disease and kidney and testicular cancers.