About 45 protesters were dressed in protective suits and wearing gas masks outside the ECan office on Tuam St on Thursday morning.
Some of them were invited into an ECan meeting on at the time where they spoke to regional councillors.
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Waste Management Ltd has applied to expand its current site and operate a waste stabilisation and treatment facility at 305 Marshs Rd in Prebbleton.
In its resource consent application, it said the plant will treat and manage items, such as pesticides, acid, used tyres and other types of rubbish.
The protesters also delivered a letter directed to ECan councillors and staff, asking for the application to be declined.
"We are prepared to go to judicial review to allow ECan to correct mistakes most people regard as obvious and we ask that you not oppose our request for this decision to be overturned and we can all therefore go back to the start and conduct this application fairly and fully," the letter read.
ECan also granted three consents for its construction and discharge of stormwater in March 2019.
The consent application currently with ECan is only for activities resulting in discharge to air.
It ran a limited notified consent process, which meant only people within 500m of the proposed facility had a chance to have their say.
The plan has prompted a major backlash from nearby residents who are calling for wider consultation.
