Customs officers and police have seized more than 65,000 litres of fuel from commercial premises in County Armagh.
Laundered fuel is filtered to remove the government marker
The officers said they were targeting non-compliant fuel distributors and seized green diesel, derv and petrol from storage tanks and a fuel tanker containing green diesel and kerosene.
Meanwhile, Customs and Excise has released details of what it described as the dismantling of "one of the largest fuel laundering plants ever discovered in Northern Ireland".
It was discovered last Wednesday in farm buildings at a rural location in the Jerrettspass area outside Newry, County Down.
It had the capacity to process about 175,000 litres of diesel per week - an annual tax revenue loss of more than £5m.
Customs head of detection in Northern Ireland, Colin McAllister, said: "This is one of the largest laundering plants that customs has uncovered in Northern Ireland, capable of laundering over 9 million litres of fuel per year.
"This would mean a potential duty loss of £5m - money that could be better spent on public services."
No arrests have been made in relation to either operation, but customs said inquiries were continuing.
Red diesel is a marked fuel for use in agricultural machinery and not for use in road vehicles.
Green diesel is the Republic of Ireland equivalent of the UK's red diesel.
Laundered fuel is red (or green) diesel, which has been filtered through chemicals or acids to remove the government marker.
The chemicals and acids remain in the fuel and damage fuel pumps in diesel cars.