A diesel laundering plant – with the potential to evade almost £2.5 million a year in duty – was discovered in south Armagh on Monday.
A Customs-led operation, in conjunction with the PSNI, saw a residential property in Cullyhanna searched.
Equipment, believed to have been connected to a diesel laundering plant, was found in outbuildings.
An 18-year-old man was arrested and later released on bail.
Customs officials said the plant was capable of producing in the region of 5.8 million litres of illegal fuel annually.
The operation saw nearly four tons of toxic waste being removed from the scene.
In addition, two vehicles, as well as 11,ooo litres of ‘red diesel’, were also seized.
HMRC says enquiries are continuing.