Officers from both Revenue and Customs and the PSNI carried out raids at nine premises in Co. Tyrone and Co Antrim in a 24-hour period.
Four men were arrested in Co Tyrone and are currently being questioned about the discovery of the plants
The searches took place in Coalisland, Sixmilecross, Carrickmore, Pomeroy and Dungannon and enabled officers to seize 70,000 litres of fuel, five vehicles, bleaching agent, tanks and pumping equipment.
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"Transporting fuel in this way poses a risk to anyone in the vicinity with illegal profit the sole motivation," said John Whiting, of HM Revenue and Customs.
"The fact that unsecured tanks full of flammable fuel are being driven around on our roads shows utter contempt for the safety of other people."
According to Mr Whiting, the dumping of by-products from the laundering process could cause severe damage to farmland and water systems.
"Taxpayers and local ratepayers would then have to pay for the clean up and disposal costs," he added.
If sold, the fuel that was seized during the raids would have cost £3.5m in lost revenue.
(NS/GK)
