Four Queensland government workers were hospitalised while investigating an underground coal gasification plant at the centre of serious pollution allegations.

Documents obtained by the ABC reveal the environment departmental investigators suffered suspected gas poisoning while testing soil at the site of the Linc Energy operation at Hopeland, west of Brisbane.

Four government workers were hospitalised during an investigation.

One of the workers said he was nauseous for several hours and his blood tests showed elevated levels of carbon monoxide.

An expert study commissioned by Queensland's environment department, also obtained by the ABC, says gases released at the plant have caused the permanent acidification of nearby soil.