The damaged pellets were then trucked 1000km from the West Coast to Marton.
Documents from the Environmental Protection Authority said the 1080 pellets were broken down at the landfill, and spread in thin layers, before household rubbish was piled on top, then compacted and topped with soil.
The Outdoors Party, which has called for an immediate stop to all aerial toxin drops, believed that was the problem.
The party's co-leader, David Haynes, said it was a woefully inadequate way to dispose of toxic waste.
"My main concern is to take a toxin and burying it in a municipal landsite where farms are raising food, possibly near some sources of water, so there is a possible risk of leaching, and also at the depth it's buried at you can almost guarantee that there will be little, if any bio-degradation."
Richard Bowman, a former biosecurity manager with Southland Regional Council, was part of a monitoring study in 1996 after 12 tonnes of 1080 bait was put in a landfill in Winton.
He said the council studied the environmental effects for 14 months and found it didn't appear to pose any significant risk to public safety or the environment, providing the site was not disturbed and natural breakdown processes were able to continue.
"In those particular conditions, which was quite damp and full of all sorts of organic material, the 1080 in the cereal bait broke down relatively quickly to the point where I think that after a year, or a year and a half there was no detectable 1080 in the immediate vicinity of the disposal site and we understood the actual material that had been disposed of had degraded to the point where it was quite neutral."
However, Bowman said 1080 bait decomposed faster when it was buried closer to the surface.
"If it was buried in a position, either in or slightly above the water table in a relatively damp situation the biological processes and the chemical processes are likely to work faster near to the surface rather than if it was buried 10 or 15 metres deep, because it's just the rate of chemical reactions and biological activity would be higher where it was warmer and closer to the surface."
Documents from the EPA said because of the way the 1080 cereal bait was disposed of in the Marton landfill it was not clear how deeply it was buried.
The EPA's acting general manager for hazardous substances and new organisms, Clark Ehlers, said 1080 must have its approval before it can be imported, manufactured and used in New Zealand. This also includes how it is disposed of - which is outlined in the Hazardous Substances Disposal Notice 2017.
"Enforcement of the rules set out in the Disposal Notice at a place of work, such as a landfill, is the responsibility of WorkSafe," he said in a statement.
"Questions about the choice of location for disposal of 1080 should be directed to the Department of Conservation.
"Note that disposal of hazardous waste at a landfill is allowed if the landfill is approved to receive hazardous waste such as 1080 through a resource consent provided by the local council."
Horizons Regional Council in Manawatu said there were no conditions prohibiting 1080 from being dumped in the Bonny Glen Landfill but officers were completing a routine assessment of the landfill's compliance with resource consent conditions.
-RNZ
