Three fuel-laundering plants capable of producing 26 million gallons a year have been discovered in Co Armagh.
The illegal manufacturing operations were found within a short distance of each other in the Cullaville area.
No arrests were made and inquiries are continuing as to whether the plants are linked.
The same volume of fuel sold legally would generate £18 million a year in revenue for the Exchequer.
One of the plants is understood to be one of the biggest raided in recent times. Hidden in a shed at the rear of a private address, its design was said to be relatively sophisticated, incorporating timers.
Police officers and HM Revenue and Customs staff were involved in the operation.
More than 100 fuel-laundering plants have been raided in Northern Ireland in the last three years.
But the authorities have faced criticism over the fact that no-one has been jailed for the crimes.
Last December Justice Minister David Ford introduced legislation that may increase the likelihood of custodial sentences in the future.
The law change handed the Director of Public Prosecutions in Northern Ireland the power to appeal over sentences for excise evasion through fuel and tobacco smuggling on the grounds of undue leniency.