The stolen lorry was discovered on Friday along the Dublin Road outside Castlewellan. Councillor Clarke visited the scene on Saturday morning and said some of the cubes inside had already started leaking onto the roadside.
The PSNI, Northern Ireland Environment Agency and Down District Council have all been notified of the incident.
Councillor Patrick Clarke said: “The dumping of illegally laundered fuel waste is costing ratepayers thousands of pounds every year by Councils having to remove the waste and these sums are coming straight from the rates of local people.
“I am very perplexed that people would have deliberately left a curtain side lorry unit which is full of cubes holding illegal laundered fuel waste on the main Dublin road outside Castlewellan which is a main route to Kilcoo and Newry.
“There was absolutely no regard whatsoever for the environment or the public health and safety of local residents living near the Dublin Road where the illegal laundered fuel waste has just been dumped on Friday, May 17.
“The cleanup operation will now be the responsibility of either Down District Council or other public health statutory agencies, who will be left with the task of safely removing and disposing of this toxic waste which will ultimately cost the ratepayer.
“There is obviously a market for cheap petrol and diesel due to the present high prices at the petrol forecourts, but people unknowingly purchasing cheap fuel are only lining the pockets of criminals who are laundering illegal fuel then are dumping the fuel waste.
“I would appeal to anyone who seen any suspicious activity along the Dublin road outside Castlewellan on Friday or have any information regarding the dumping of the stolen curtain side lorry unit full of illegal laundered fuel waste to report to the PSNI.”