The Melbourne factory that exploded in a chemical inferno on Friday morning is linked to another factory that went up in flames last August, according to the findings of an Environmental Protection Authority investigation.

Key points: Last week's Melbourne factory fire has been linked to an "alleged criminal operation"

Last week's Melbourne factory fire has been linked to an "alleged criminal operation" Internal EPA documents connect the Campbellfield fire to last year's West Footscray factory fire

Internal EPA documents connect the Campbellfield fire to last year's West Footscray factory fire The Hells Angels have also been linked to the network behind the fire

The environmental regulator is also expecting to find more unlicensed dumps.

The factory blaze in Campbellfield last week choked surrounding suburbs in Melbourne's north with thick smoke and caused nearby schools to close.

The ABC can reveal that the EPA's investigation, launched after last years' West Footscray factory fire, has identified "an alleged criminal operation" involving over a dozen outer-suburban Melbourne sites where vast amounts of hazardous chemicals are being stored, in most instances without a licence.

The ABC can also reveal it is feared that toxic chemicals may have been dumped in sinkholes at a property in western Victoria, and that the company whose factory caught fire last week has transported hazardous waste in rental trucks that are not licensed to carry the dangerous goods.

Code-named Operation Hydrogen, the EPA investigation led the regulator to numerous locations in the Melbourne suburbs of Craigieburn, Epping, Tottenham and Campbellfield — including the Campbellfield site that exploded in flames on Friday morning — where it has now found over 35 million litres of hazardous chemicals.

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Internal EPA documents obtained by the ABC show the investigation identified a number of associates of the West Footscray factory tenant, Graeme Leslie White, were involved in the operation, including Bradbury Industrial Services, which was based at the Campbellfield factory that burnt down last week.

"White has advised EPA officers that he has a commercial relationship with Bradbury. Anecdotal evidence suggests that White probably also has a personal relationship with one or more employees of Bradbury," the documents say.

Bradbury Industrial Services offers storage and disposal services for hazardous waste. ( ABC News: Dylan Anderson )

The alleged criminal operation "involved the acceptance of chemical waste from various businesses for a fee, which is then stockpiled in warehouses rented by White or Bradbury."

Despite extensive efforts, the ABC has been unable to contact the Bradbury individuals named in the EPA documents. The company's office phone number now rings out and emails bounce back undeliverable.

Hells Angels links to network investigated

Mr White was jailed last week over a large haul of illegal weapons discovered when police and the EPA raided his western Victorian property at Kaniva.

Graham White photographed outside one of the sites inspected by the EPA. ( ABC News )

The EPA documents state there is anecdotal evidence he is "linked to the Hells Angels outlaw motorcycle gang."

The investigation raises the prospect that networks involving convicted criminals, prepared to flout licence laws, may have infiltrated the lucrative waste disposal market, hoarding enormous quantities of dangerous toxic chemicals in suburban Melbourne, unregulated and in some instances beyond the knowledge of the regulator.

Bradbury occupies four warehouses the EPA identified, in addition to the Campbellfield warehouse that burnt down on Friday.

Those four sites were among the unlicensed group. The site of Friday's fire had been licensed, but that licence was suspended after the EPA found in late January it contained three times the volume permitted under its licence.

Toxic waste carried in rental trucks

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 22 seconds 1 m 22 s About 100 firefighters responded to the blaze.

A former employee of Bradbury, who did not want to be identified, told the ABC the company hired trucks from rental agencies to use in the transportation of chemicals, despite those vehicles not being licensed for such a purpose.

He said when EPA inspectors visited Bradbury's premises, they did not inspect the trucks.

The former employee also said that many of the containers of chemicals that Bradbury's drivers would collect from customers were unlabelled, and that Bradbury employees had no way of knowing what was in them.

"How waste is disposed of is relying at almost every step on honesty and complete information, whereas it was very rare that you would actually get that from the people we were picking it up from," he said.

"Literally hundreds of times you would be sent somewhere to pick something up, and you wouldn't know what it was, they wouldn't know what it was or it would be something different from what you were told it was."

In a statement to the ABC, the EPA said it took such allegations extremely seriously.

"The safety of the public and the environment are our first concerns. However, the Bradbury fire is the subject of multiple investigations by the coroner and arson squad.

"EPA cannot comment on any matter that could be pertinent to those investigations in case it in some way jeopardises the outcome."

The ABC has been told that only days before last week's fire started, the EPA demanded the company hand over documents and records relating to the chemical stockpile.

The EPA documents show the investigation also identified eight unlicensed sites occupied by Mr White where chemicals were stored.

Chris Baldwin is "believed to have links to the Hells Angels," according to EPA documents. ( Supplied: Channel Nine )

"[T]he financial benefits for White and his associates probably total millions of dollars.

"Based on the volumes of chemicals found and the ongoing discovery of warehouses containing stockpiled chemical waste, it is probable more sites containing stockpiled chemical waste will be identified by the authorities."

The EPA documents state that Shepparton accountant Christopher Baldwin, the owner of the West Footscray site leased by Mr White is also "believed to have links to the Hells Angels."

The ABC has previously reported that Mr Baldwin, who was raided last year by the Tax Office during a phoenixing investigation, is linked to senior Hells Angels through company directorships and shareholdings.

The documents do not link Mr Baldwin to the waste operation, but detail a number of corporate connections between Mr White's companies, Mr Baldwin's former accounting firm, and the West Footscray factory address.

Repeated attempts by the ABC to contact Mr Baldwin in the past have gone answered.

Drums found with 'blue liquid'

The coroner has launched an investigation into the Melbourne factory blaze. ( ABC News: Dylan Anderson )

Last July, EPA inspectors accompanied by officers from Victoria Police and the Australian Federal Police visited a property owned by Mr White near the town of Kaniva in western Victoria and found 26 sites where vegetation had been cleared.

At one of these sites they discovered a number of drums containing a blue liquid.

The EPA inspectors also tested soil near a number of sinkholes on the Kaniva property, finding traces of the toxic and flammable solvents toluene, ethyl benzene and xylenes. It is not known whether any chemicals have been deposited in these sinkholes.

Last month, the EPA issued Bradbury with a "show cause" notice.

"Bradbury responded to the show cause letter with a generic explanation of the services they provide to the waste industry and details of their future plans for extension. No explanation was provided as to how the non-compliance of their licence was to be addressed," the documents state.

The EPA then suspended Bradbury's licence.

Two workers from the Campbellfield warehouse were hospitalised following the fire. One, who was carrying a barrel of chemicals at the time the fire struck, has serious burns, including to his face.

Victorian Coroner Darren Bracken has announced he will investigate this week's fire. The coroner is already investigating the West Footscray fire. A coroner can investigate a fire regardless of whether a person has died.