Review of use of Overseer as a regulatory tool

20 Apr 22

Overseer was developed as a tool to increase efficiency and productivity for farmers, but in the last decade has been increasingly relied on by regional councils as a regulatory tool to manage nutrient discharges.

In 2018 the Parliamentary Commissioner for the Environment undertook a comprehensive review of Overseer, identifying a number of shortcomings and issues associated with its use in a regulatory setting. He recommended a series of further investigations be undertaken.

A Science Advisory Panel was appointed by the Ministry for Primary Industries and Ministry for the Environment to undertake a further review, and its report was released in mid-2021. The Panel identified particular concerns with the model, including:

  • its use of long term (30 year average) climate data, without the ability to model intermittent events such as heavy rainfall or drought which are critical drivers of nutrient losses;
  • its use of homogenous soil profiles, limiting the model’s ability to account for variability in water and nutrient distribution;
  • inadequate capture of overland flow of nutrients;
  • ammoniacal nitrogen (found in organic matter like animal waste) and organic nitrogen dynamics are omitted from the system; and
  • deep-rooting plants are not accommodated.

The Panel also raised concerns that the model was developed without engagement with Māori, and the principles of Te Tiriti o Waitangi are not included or reflected in the system.

The Panel concluded that it did not have confidence that Overseer’s modelled outputs were accurate or that Overseer could reliably estimate whether changes in farm management would reduce or increase nutrient losses.

Government response

The Government response to the Panel’s findings was released concurrently with the review. It identifies potential implications of the findings in the short term, including dealing with the uncertainty the review poses for existing farming operations and reliance on Overseer, and also addresses options moving forward.

Existing consents and plans

The most immediate concern is the impact of the Panel’s findings on existing resource consent monitoring and anticipated renewal of existing consents, as well as general planning and regulatory functions.

The Government has highlighted its commitment to continued availability of Overseer while options to improve the system are assessed. The response notes that the Panel generally found inaccurate representation of loss through overland flow and drainage, and therefore suggests that in a farming practice where nitrogen leaching is the dominant form of nitrogen loss, Overseer may still be appropriate.

However in other scenarios, the response suggests councils should use additional information on nitrogen losses, and suggests guidance on this be developed in the interim. This could include:

  • alternative data and modelling to be used where it is available either in addition to or in place of Overseer;
  • consents issued should provide for the future potential use of a redeveloped system;
  • until any redevelopment occurs, councils are legally required to implement the provisions of their regional plans and resource consents, but within the extent of those bounds room should be found to:
    • use all available information, and exercise caution with Overseer, when imposing consent conditions, monitoring compliance, and carrying out enforcement;
    • new consent conditions should allow for compliance to be demonstrated using ‘multiple lines of evidence”, i.e. different types of evidence; and
    • a ‘best information available’ approach should be considered when assessing compliance with existing consents and plans.

National Environmental Standards for Freshwater

The National Environmental Standards for Freshwater (NES-F) include a number of standards where quantitative information on nitrogen and other contaminants (e.g. phosphorus, sediment and E. coli) is required to demonstrate compliance, quantifying the amount of nitrogen leaching to water as a result of nitrogen application to land.

As above, the government has recommended alternative evidence be used in addition to Overseer, to assess activities against the requirements of the NES-F.

National Policy Statement for Freshwater Management (NPS-FM) – new or modified regional plans

The NPS-FM requires existing regional plans to be updated or new plans prepared by 2024 to reflect policies in the NPS-FM. To meet this timeframe, most regional councils will either be starting or partway through catchment modelling, or will have already completed this process, with at least some utilising Overseer to do this. There are limited alternative tools for catchment modelling and therefore continued use of Overseer will be necessary to meet the 2024 deadline.

Advisory Note

The Ministry for Environment has released an advisory note for regional councils on how to manage activities following the government response (Advisory Note). The Advisory Note is guidance only. Key takeaways include:

  • reinforcement of the ‘best information available’ and ‘multiple lines of evidence’ approach: the Advisory Note suggests adopting this where possible within existing policy and consenting frameworks;
  • encouragement for regional councils to incorporate the findings of the Panel’s review into current plan review processes, whilst ensuring any alternative approaches adopted give effect to the National Policy Statement for Freshwater Management (NPS-FM);
  • councils may wish to request additional information to support its decision-making, such as:
    • detailed land use and proposed land use descriptions;
    • detailed descriptions of the receiving environment, including all components of the local freshwater system and how they are connected and identifying nearby water users and sensitive receptors;
    • monitoring plan including actions or responses proposed; and
    • a full assessment of the range of relevant contaminants; and
  • for applications for consent under the NES-F, the Advisory Note suggests additional information other than Overseer modelling will be needed to provide certainty that requirements relating to contaminant load and synthetic nitrogen limits are being met.

Next steps

The Government has proposed an initial list of options to investigate and put in place either individually or in combination:

  • creation of a new risk index tool, potentially using elements of Overseer; and
  • development of a next generation Overseer to address the issues raised by the Panel and to ensure that it is fit for purpose as a tool to use in appropriate regulatory settings; and/or
  • greater use of controls on practices and inputs to manage nitrogen leaching (including through amendment to the NES-F); and/or
  • a completely new approach to understanding and managing diffuse nutrient loss risk, which may include near real-time water quality monitoring, a tool to base nutrient loss risk on characteristics of land, and a new nutrient loss model.

These options are currently under investigation, with results and decisions for regulatory tools anticipated to be available from mid-2022.


PDF version: here.

This article was included in Edition 5 of our rural newsletter – Rural. which you can read here.

For more information contact:

Sarah Eveleigh