Four men have been arrested after the discovery of three illegal fuel plants capable of producing 15 million litres a year.
Customs officers discovered two separate diesel laundering plants at farm buildings last week.
The first was found at a building in the Crossmaglen area of Co Armagh on Thursday, while the second was uncovered in the same area the next day.
Elsewhere 2,500 litres of illegal fuel was seized from a station forecourt in Co Tyrone.
A third plant was also dismantled following the search of a house in Dungannon.
The discoveries are the latest in over two dozen fuel laundering plants uncovered by customs over the last year.
John Whiting of HM Revenue and Customs said some 22 tonnes of waste had also been removed from the sites.
"Our activity over the last week has not only stopped a large amount of laundered fuel entering the legitimate market, but 22 tonnes of toxic waste was also removed from these three sites," he said.
"This chemical sludge would otherwise have been dumped indiscriminately in our countryside.
"Buying illicit fuel not only funds crime, but also supports and encourages this dangerous activity within our communities."