Because the incident is still under investigation, Mellor said he couldn't provide any details on where the plastic came from or what form, size or shape it was in.
The regional council is also working with the three identified sites and Mellor encouraged members of the public that smell the sulphur-like odour, to call the council's Pollution Hotline and provide as much descriptive detail as they can so it has the best opportunity to pin-point the source.
As part of the council's investigation, it will be considering any environmental impact.
"If anyone finds, sees or smells pollution, please report it to our 24/7 Pollution Hotline on 0800 884 833," Mellor said.