This stockpile of potentially toxic and highly flammable materials could be more than 10 times larger than originally estimated, forcing emergency services and regulators to radically revise their safety, handling and disposal plans.

This is an estimate of the clean-up challenges contained within the eight warehouses in Epping and Campbellfield that have been identified so far as dump sites, so it could increase as other illegal sites are discovered.

Sources say the cost of cleaning up what is now undoubtedly the biggest illicit dumping operation in the city’s history could reach $50 million.

Authorities had originally estimated by sight in late 2018 that the properties contained about 1.5 million litres of liquid waste, including solvents, paints, detergents, aerosols and cleaning chemicals. When discovered, the warehouses were packed almost floor-to-ceiling, wall-to-wall with industrial-size plastic containers and drums.

The new estimate, which is based on calculating the potential total capacity of the warehouses, is being used as a basis for designing new, worst-case emergency response plans.

WorkSafe, which is leading the containment and clean-up effort, declined to comment on the nature of the safety plans or its risk assessment of the situation for operational reasons.

“Emergency services, government agencies and local councils are aware of this and have updated their contingency plans accordingly,” a WorkSafe spokeswoman said.

But documents obtained by The Age show authorities initially rated the consequences of a fire or spill at one of the sites as “MAJOR” and representing a “danger to life” in their emergency response plans created when the stockpiles were first assessed in late December.

They included informing nearby schools, hospitals, businesses and transport authorities  about the risks of the clean-up and developing individual emergency plans.

Looming large in the risk assessment is the threat of another massive industrial blaze similar to the one at the West Footscray factory in August last year, which burned for three days and forced the closure of schools and businesses.

That property and the sites in Epping and Campbellfield were allegedly all run by Wallan-based businessman Graham Leslie White.

It remains unclear whether taxpayers or Mr White will ultimately foot the bill for the clean-up.

The dramatically revised estimate in the volume of chemicals contained at the sites - which is the equivalent of seven-and-a-half olympic-size swimming pools - came after technicians were able to enter the properties.

When the illicit dumping operation was exposed in late December 2018, authorities faced serious problems entering the warehouses because of the tightly packed pallets of 44-gallon drums and industrial-size plastic bins.

“The drums were packed to the rafters and went from wall to wall,” a council inspector wrote. “Given the number of drums and containers we were not able to move through the factory.”

The assessment of the sheer scale of the dump site only became clearer as WorkSafe began the complex process of sorting and clearing some of the warehouses.

Three more dump sites were recently discovered in Craigieburn and another in Campbellfield on Friday, although it is still unclear how much waste has been stockpiled and who is responsible.