Scientists warn the impacts of the West Coast landfill "eco disaster" could be felt for generations and potentially "tens of thousands" of old dumps around the country also posed environmental risks.
Tonnes of recycling and rubbish has been washing up on previously pristine Westland district beaches after an old Fox Glacier landfill breached following a major storm.
The buried dump was scoured out by the flooded Fox River, with rubbish washing into the Cook River and carried to the sea.
It comes a little over a year after high seas from ex-Cyclone Fehi tore through an old coastal rubbish dump at a Greymouth beach, spilling tens of thousands of plastic bags and other items onto the coastline.
Associate Environment Minister Eugenie Sage said she would be seeking information on other landfills across the country exposed to flooding risks.
Niwa's Dr Joshu Mountjoy said some of the rubbish would likely end up at the bottom of a deep canyon off the coast, which was home to key marine ecosystems.
"We know from recent studies sediment from rivers is being moved into canyons on the West Coast and it is likely that a component of the Fox River landfill waste will end up in the deep ocean by the same processes," Mountjoy said.
Offshore from the Fox River, the Cook Canyon came within at least 4km of the shoreline and was a major pathway for transporting sediment from rivers to the deep ocean.
The end of this canyon had not been found but was deeper than 4000m.
Mountjoy said a study on canyons offshore of Sicily, published in the journal Scientific Reports, found huge amounts of rubbish in water depths up to 1000m transported there in response to flash flooding.
These canyons held key marine ecosystems and some of the "most concentrated biomass on earth and supporting important fisheries"
Clean-up to take generations
Auckland University of Technology senior lecturer Dr Jeff Seadon said the clean-up would be a big job with impacts felt for generations.
"When it gets into the oceans, currents can carry it thousands of kilometres from its origin as we have seen with plastics in the marine environment.
"By the time it gets back to shore, beachcombing is really the only way to clean it up.
"These wastes can be hazardous, so cleaning them up requires specialised knowledge and equipment to avoid injury or adverse health effects."
Hazardous wastes included household and industrial chemicals like paint, garden pesticides, pharmaceuticals, personal care products, waste oil, batteries and treated timber that could all leach into surrounding soils, waterways, groundwater and the ocean.
Those wastes could poison plants and animals, or be absorbed and passed up the food chain to come back to us in a more concentrated form.
Seadon said New Zealand's modern landfills were "world class", capturing a high percentage of methane emissions and had liners that ensured leachate containing hazardous waste was not able to pass into the environment.
They were also sited away from coastlines and waterways to ensure minimal damage in storms and other natural hazards events.
However, past landfills were simply "dumps", with no liner, no methane collection nor cover, allowing water to pass through allowing contaminants to spill into the environment.
They were also often located by the seaside or near waterways.
Filling in a bay to eventually provide parks or a valley to get the waste out of sight was also a common approach.
Seadon said the number of these "dumps" was unknown, but there were possibly tens of thousands of farm dumps where farm waste was dumped in a valley and covered over, allowing hazardous waste to get into groundwater or trickle into nearby streams.
"Storm events, like the recent one at Fox Glacier on the West Coast, can uncover these dumps and the waste then flows away, often to the nearest marine environment.
"From there it can be carried out to sea or along the coast to wash up wherever the currents carry it."
Remediating these dumps is expensive, and the cheap solutions of decades ago have "come back to cost us greatly to fix the problem".
Seadon said a simple fix was to reduce waste, particularly in construction and demolition, and organics, which together made up two thirds of the country's landfill waste.
Recovering materials instead of demolishing buildings, and reducing waste on sites would have a big impact and help move to a circular economy.
Plastic bags also had an environmental impact disproportionate to their weight, as evidenced by the bags strewn across West Coast beaches.
Seadon said more alternatives to disposal for these items needed to be explored, including in lightweight concrete and roading.
Sea-level rise posing extra risk
A Local Government New Zealand Report assessing the impacts of climate change found the Auckland region held 88 old landfill sites which would be exposed with 0.5m of sea level rise. Otago, Nelson, and Canterbury also had multiple closed landfills at risk.
A LGNZ spokesman said the Auckland situation was quite different to that of the West Coast however, as most of its landfill sites were inland.
"It is just that Auckland's land mass is quite flat."
Forest & Bird conservation advocacy manager Jennifer Miller said there were likely to be hundreds more old landfills near rivers and lakes, also at risk from climate change impacts like increasing storm events.
The Ministry for the Environment had a responsibility for making sure councils were not consigning their residents and landscapes to more of these preventable ecological disasters, she said.
"The Ministry for the Environment must provide clear guidance to councils on what their responsibilities are, and central Government must resource this regional work.
"These problems cannot be dealt with by 'bringing in dozers' as the Westland mayor seems to think, nor building retaining walls. Councils need to take action on adapting to climate change, including by protecting old landfill sites from erosion."
"Restoring nature is a key part of preparing for climate change because healthy sand dunes, wetlands, and forests will slow water and reduce flooding and scouring effects."