Long before Melbourne's northern suburbs were choked with acrid smoke from a mammoth factory fire earlier this month, workers inside were finishing their shifts covered in toxic sludge and struggling to breathe.

Key points: Workers have lifted the lid on conditions inside the Melbourne factory that was engulfed in a mammoth blaze in April

Workers have lifted the lid on conditions inside the Melbourne factory that was engulfed in a mammoth blaze in April Some employees said they suffered physical and respiratory problems as a result of being exposed to chemicals

Some employees said they suffered physical and respiratory problems as a result of being exposed to chemicals Workers say the company was advised about EPA inspections ahead of time and hid problems from the regulator

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It's now known the factory was home to a vast illegal chemical waste dump — one internal EPA documents allege was linked to a criminal network responsible for more than a dozen similar illicit waste dumps around Melbourne.

But multiple employees at the Campbellfield company describe a warehouse in the lead-up to the fire where chemical drums were not correctly stored and where workers wearing inadequate safety equipment were frequently covered in chemicals that caused physical and respiratory problems.

"I had burns all over my body due to handling some chemicals. They did not tell me what chemicals they were," said one worker, Muththukirishnan Karththikeyan.

"Sometimes, it burns. If I tell them that I got burns from the chemicals, they would say 'that's how it is. It'll just be like that for a short period of time,' and then they would just apply a cream."

Employees told the ABC that the company — Bradbury Industrial Services — were able to get away with it, as managers were forewarned days ahead of EPA inspections and would order workers to hide chemicals to deceive inspectors.

"They would tell us that EPA is coming a day or two prior to EPA coming. They took away all the things from there to another store. They transferred using a truck," said another worker, who did not want to be named.

Employees also told the ABC they would only be given appropriate safety clothing during inspections by the EPA, but would otherwise have to supply their own basic cotton or polyester uniforms.

Muththukirishnan Karththikeyan said workers would only be given appropriate safety clothing during EPA inspections. ( ABC News: Jeremy Story Carter )

"If EPA comes, they would make the company seem safe, only that day," Mr Karththikeyan said.

"If EPA is coming, on that particular day, all safety goggles must be worn, and a mask must be worn. Protective outfit would also be provided. Everything has to be worn only on the day EPA comes."

Bradbury eventually had its license suspended for storing three times the amount of waste it was entitled to and was being investigated in the days prior to the massive fire on April 5.

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.

The fire shut down nearby schools in Melbourne's northern suburbs and families were urged to stay indoors, while there were reports of chemical drums being sent flying dozens of metres into the air as a result of the explosion.

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In a statement, EPA chief executive Cathy Wilkinson acknowledged the regulator did flag inspections in advance, but in the wake of the two warehouse fires, it was increasing its number of unannounced inspections.

"EPA conducts a combination of announced and unannounced inspections with an increased focus on unannounced inspections," Dr Wilkinson said.

"EPA is investing $5.5 million to switch to a fully GPS electronic waste tracking system to better record the production, movement and receipt of prescribed industrial waste which will provide improved quality data, helping us to detect potential risks and intervene earlier.

"With the Bradbury situation under multiple investigations, EPA is limited about what can be discussed."

Worker has face burned, another has torso blistered

Vignesh Varatharaj suffered burns to his face and ear after a chemical drum he was working on exploded. ( ABC News: Jeremy Story Carter )

Many of the workers at the Campbellfield premises are Sri Lankan Tamils and speak limited English.

One worker — Vignesh Varatharaj — was badly injured and had his face burned on the day of the fire after he says a chemical barrel exploded next to him.

A crowdfunding page has so far raised over $24,000 to help with his medical costs.

A photograph supplied to the ABC appears to show a separate worker's torso covered in blisters, which his colleagues said was a result of him being exposed to chemicals while working prior to the factory fire.

Employees said this worker's stomach became blistered after being exposed to chemicals at the factory. ( Supplied )

"All the chemicals caused blisters all over his body. They didn't take him to the hospital. He went on his own," Mr Karththikeyan said.

"I think they told him at the hospital that the chemical was the problem, which caused an allergic reaction and hence this resulted. When he told this to the manager lady, the boss told him 'it was not caused by the chemicals. Your body has allergy.'"

Vignesh has had to undergo medical treatment after suffering chemical burns. ( ABC News: Jeremy Story Carter )

Employees said they were too afraid to complain about the conditions at the warehouse because they risked losing their jobs.

"You can't complain like that there. You can't say that to them. If you tell them, they would say that they would fire you from work. They would scare us by saying that they would fire us from work if we talk too much," said Mr Karththikeyan.

Some employees said they regularly finished shifts with physical and respiratory difficulties after being covered in chemicals while working. ( Supplied )

He provided the ABC with photographs of him and a colleague during a shift after having their skin and clothing caked in toxic sludge.

He said if workers complained about having difficulty breathing while being exposed to chemicals, they were told to simply take chemical drums outside and continue working.

Criminal links swirl around factory's owners

Last year, Bradbury was forced to back-pay a number of workers who were found to have been underpaid.

Over $24,000 has been raised through crowdfunding to help Vignesh cover his medical costs. ( ABC News: Jeremy Story Carter )

It is understood that the former manager of Bradbury, Mark Anderson, was convicted in 2007 in Victoria of stealing more than $1.3 million from a New South Wales car dealership of which he was the managing director. It appears that he was known by another name when he was prosecuted.

The EPA documents obtained by the ABC suggest that Mr Anderson also has links to greyhound trainers in NSW who were banned after their dogs tested positive for illicit substances.

The documents also suggest that some clients of Bradbury, who paid the company to remove and dispose of chemicals, may have known Bradbury was incorrectly storing the products.

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

The documents also list a number of clients, which include medical laboratories and some of Australia's biggest paint manufacturers.

A number of industry sources have told the ABC that Bradbury was undercutting other companies by offering dramatically lower prices to dispose of chemicals.

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

They said other players in the industry wondered how Bradbury could have lawfully disposed of the quantities of chemicals the company was taking in.

One man allegedly linked by the EPA to Bradbury, Graham Leslie White, was recently jailed for illegally possessing weapons, including a loaded machine gun.

It is also suspected that White was dumping toxic and flammable solvents at a property near Kaniva, in the west of Victoria.

Representatives from Bradbury did not respond to the ABC's attempts to seek comment.