There are at least 51 inactive landfill sites in the Bay of Plenty but the environmental risks from some of them remain unknown, for now.
Today, Environment Associate Minister Eugenie Sage announced a multi-agency response to identify risks from legacy landfills. The move was prompted by an extreme storm in March which resulted in significant exposure and erosion of an inactive landfill at Fox River.
"This project, which involves key agencies from local and central government, will help us to understand what needs to be done to avoid another Fox River," Sage said.
In the Bay of Plenty, most inactive sites recorded with the Bay of Plenty Regional Council were located within Tauranga's CBD.
Both the regional and Tauranga City Council believe there may be more undocumented landfill sites and the newly-announced response will help uncover them.
Regional council's regulatory compliance manager Stephen Mellor said there were likely to be more than 51 as some sites were still being assessed.
"It's also likely there are sites unknown to us where waste has been buried in the past."
The sites were a mixture of historic sawmills, old private farm dumps and municipal landfills.
Mellor said most were not at risk of being exposed through erosion and because of this the regional council had not carried out many formal risk assessments.
Under the Resource Management Act, the regional council is the regulator for such sites but not responsible for remediating or repairing environmental damage if a vulnerable landfill is breached in an event or by a process. This responsibility sits with the owner.
Mellor said his team welcomed the Government announcement and looked forward to "working with the Government and the community to better understand the risks from existing and legacy landfills".
Tauranga City Council's manager of sustainability and waste Malcolm Gibb also welcomed the announcement.
While new landfill waste was dispatched to and processed at Tirohia and Hampton Downs, historic landfills such as Te Maunga and Cambridge Rd were closed but still managed.
Both Te Maunga and Cambridge Rd were subject to erosion from extreme stormwater, however because Te Maunga was landlocked there was less risk, Gibb said.
There were a number of other sites known to have been used as a dump during the last century, such as in Glasgow St, Fraser St and Graham Park.
"It is unknown how large these sites are as they are not documented. There have been no reports of leachate or issues with ground contamination or instability for these sites and there is no formal council monitoring or management.
"There are very likely more, undocumented sites that were used in the past century as a dump. The project that was announced by the Ministry for the Environment will undoubtedly help us uncover more, previously undocumented sites."
There are four closed landfill sites in the Western Bay of Plenty.
Rotorua Lakes Council did not respond to queries before deadline .
How Bay of Plenty Regional Council monitors landfill sites
* There are eight open consented landfills and 12 closed landfills spread throughout the Bay of Plenty. Some of these are for industrial sites, including one which uses a consented landfill for wood waste.
* The regional council receives regular reports from consent holders as part of their consent conditions. All landfills are inspected at least annually except the Rotorua, Tauriko and Ōropi sites which are inspected at least twice a year as they are the only municipal landfills in the region.
* When the regional council inspects closed landfills, it looks for surface cracking on the earth placed over the site, odour and leachate ponding.
* It also looks at stormwater quality and odour. How this is carried out depends on the nature of the site and consent.
* Active transfer stations are managed by district councils. The Te Maunga site has quarterly inspections and is managed by Envirowaste, all other sites are inspected annually.
* How often a landfill or transfer station is monitored also depends on the scale of the activity, risk to the environment and compliance history.