A secret Queensland Government report allegedly warns that hundreds of square kilometres of prime agricultural land in the state’s southeast are at risk from toxic chemicals and explosive gases.

The study – which was the subject of an in-depth ABC report this week - reportedly said that an experimental plant operated by mining company Linc Energy at Chinchilla has caused "irreversible" damage to strategic cropping land.

The documents were commissioned by Queensland's environment department, which has launched a $6.5 million criminal prosecution of the company.

They allegedly state that Linc is responsible for “gross interference” in the health of former workers, on top of the “serious environmental harm”.

Related documents show four departmental investigators had to be hospitalised after they suffered suspected gas poisoning during soil testing at the site earlier this year.

While the risks were not disclosed to nearby landholders, they may have become apparent when Linc sectioned-off a 314 square kilometre exclusion zone around the facility, in which landholders were banned from digging deeper than two metres.

On the environmental health side, consultants Gilbert & Sutherland claim that one of the techniques Linc was testing involved injecting air into underground combustion chambers at pressures that were too high.

This reportedly caused the rock around the coal seam to fracture, allowing the escape of “volatile organic compounds, which are known carcinogens, and 'bulk' gases,” the analysts said

“Not only was the pressure high enough to fracture the coal seam, it was also high enough to fracture the overburden,” the authors wrote.

“Once this fracturing had occurred, it was unsafe to ignite the coal seam.”

Former workers told ABC reporters that gas alarms sounded almost constantly during their time at Linc.

They claimed gas leaks from wellheads and from the ground gave them regular headaches and sickness.

Two workers said they were even forced to resign when their health concerns were ignored by the company.

“Well into my time at Linc I began to suffer chest pain and flu-like symptoms. These symptoms included general aches. I recall a lot of us had similar complaints,” one worker said.

“I often felt unwell on site — headaches, feeling ill in the stomach,” another said.

“When you were working at night when there was no breeze, your personal gas detector would be constantly alarming to the extent that I'd get in my vehicle and drive off site for a few kilometres before it would stop alarming.

“I just had to breathe fresh air.”

In matters that have been made public, the environment department ordered Linc to provide $22 million to clean up water and sediment in several storage dams at the facility.

The department says the dams are “likely” to be contaminated with dioxins and other pollutants.

Linc has taken the matter to the Land Court, where it appears to be arguing that the demand is unreasonable and unfounded.