Mining company Linc Energy allegedly failed to report a series of dangerous gas leaks at its experimental Chinchilla coal gasification plant in Queensland and instead covered them up, according to court documents.

Search warrants and other documents filed in the Supreme Court in Brisbane allege a series of unreported incidents at the plant, unearthed as part of a major state government investigation after tip-offs from former workers.

They include a fire that caused Linc to evacuate the site in 2007, persistent leaks of toxic gas into the air and groundwater between 2008 and 2011, and an alleged attempt by management to hide problems at the site from government officials in 2012.

Workers even had nicknames for some of the gas leaks: "Mr Bubbles" and "Puffing Billy".

Underground coal gasification (UCG) is a controversial technique involving the ignition of the coal seam, with oxygen pumped into a well to allow for the combustion of coal, which then produces synthesis gas or syngas.

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Farmers and conservationists have long been critical of UCG, warning it has the potential to poison aquifers and contaminate land.

The court documents also contain further claims that unlawful gas releases by Linc Energy affected the health of workers at the plant.

A search warrant filed in the Supreme Court alleges that staff at Linc's Chinchilla site had complained to the company of "medical conditions ... including bleeding noses, dizziness, nausea and vomiting, headaches, blurred vision and respiratory ailments" after being exposed to odours from dams at the site.

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The warrant alleges that staff had reported the incidents and related gas releases, but none was ever reported to the authorities.

"Linc has caused each of its staff to sign a confidentiality agreement, so as to prevent disclosure of the matters referred to in these particulars," the warrant states.

But it adds: "Details of the matters referred to in these particulars have been advised to Linc's board and senior management in letters of resignation from former technical and other staff, in other human resources records... by way of operator logs, and in emails and other correspondence."

Gas leaks given names by workers

The documents also show that during briefings ahead of raids on Linc's head office in Brisbane, its Chinchilla site, and two other locations in 2013, environment staff were told that "syngas is leaking from multiple locations on Linc's site at Chinchilla ... all of the gasifiers are fractured and that fracture is occurring on site. Linc is aware of this".

What is UCG? Underground coal gasification is a so-called "unconventional" means of extracting gas from coal seams that are too deep to mine. Coal is burned in situ underground and the gas produced is siphoned off through wells.

Underground coal gasification is a so-called "unconventional" means of extracting gas from coal seams that are too deep to mine. Coal is burned in situ underground and the gas produced is siphoned off through wells. It differs significantly from CSG production, which involves drawing enough water to reduce the pressure underground to release the naturally occurring methane in the coal seams.

It differs significantly from CSG production, which involves drawing enough water to reduce the pressure underground to release the naturally occurring methane in the coal seams. UCG technology was first proposed in the 19th century but was only adopted seriously after World War II in Russia and former Soviet client states.

UCG technology was first proposed in the 19th century but was only adopted seriously after World War II in Russia and former Soviet client states. Benefits of UCG include the fact that landscapes remain relatively unscathed compared with conventional mining, emissions are low and the gas produced can be used for power generation or condensed to make liquid fuels such as diesel.

Benefits of UCG include the fact that landscapes remain relatively unscathed compared with conventional mining, emissions are low and the gas produced can be used for power generation or condensed to make liquid fuels such as diesel. Disadvantages of UCG are that it is sensitive to local geological conditions, expensive to set up, and the technology is highly specialised and poorly understood by regulators.

Unreported incidents at Chinchilla allegedly include a fire caused by a clogged pipe, which the company tried to clear by increasing the pressure so much that the rock above it cracked, allowing gas to escape; unreported benzene contamination of groundwater at levels more than 60 times higher than allowed; and attempts to hide gas leaks by covering them with crusher dust.

Each of the incidents detailed in the warrants is accompanied by the comment: "This incident was not reported to the Government as required by law".

According to the warrant, during heavy rain in 2010, bubbles could be seen "penetrating the surface of the ground above Gasifier 4", equipment used to extract syngas.

"The source of the bubbles was identified by Linc staff to be carbon monoxide," the warrant states.

"Linc staff, including management, were aware of this issue, and staff gave the area where bubbling was occurring the name 'Mr Bubbles'."

The warrant later describes how during a joint site inspection by the Queensland departments of environment, natural resources and employment in January 2012, "Linc management caused staff to reduce the pressure in Gasifier 4 ... so as to conceal any leakage".

The environment department also alleged that the company and consultants working for it recorded extremely high levels of contaminants at monitoring wells on the site.

"Samples taken at the site ... demonstrated either high levels of contaminants, or levels of contaminants so high that these samples were rejected and not tested by the third party laboratory on the basis that doing so may risk damage to laboratory instruments," the warrant states.

"In particular, monitoring well M2 became notorious amongst Linc's staff, and was known as 'Puffing Billy', as a result of regular and repeated observations of the release of syngas."

Authorities always informed of incidents: Linc

The documents emerged as part of a court battle between Linc Energy and the environment department over the seizure of evidence that began in 2013. They have not previously been reported.

Linc obtained a court ruling that some of the search warrants issued to the department were unlawful because they were too broad and material seized under them was returned. The department is appealing the decision.

Linc Energy said it intended to defend the charges laid against it.

The company told the ABC the description of the 2007 fire in the court documents was inaccurate and it was "unaware of the specifics" of the incident involving benzene contamination of groundwater.

"However we can confirm that where our environmental authority requires notification to the Department for groundwater results, such notifications have always been made within the required timeframes," Linc said.

The company added it was normal industry practice for companies developing new technologies to make employees sign confidentiality agreements, to protect their intellectual property.

It said it had received no formal notifications of the symptoms allegedly described by former workers in the court documents.

"We have not received direct complaints from former employees," Linc said.

Linc Energy chairman Peter Bond this morning defended its operations, saying he could not understand why his company was being targeted.

"It's an absolute witch hunt - the charges haven't been able to be taken to the next level," he said.

"It's been a really long and difficult case for them and purely because they haven't had the evidence to back up the claims ... they're getting desperate, which is why they went public with various claims, which are in some cases seven, eight years old."

He said some workers had previously complained of ill health but that stopped in 2008 when a new health and safety program was brought in.

"I've been in mining for 32 years and I can tell you there would be every single week or other week somebody would complain of a headache - the investigation would be full," he said.

"We have acted absolutely 100 per cent professionally in every case.

"We would go right through to the end result."

The environment department said that as the matters were before the court, it was unable to comment.