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Tracking surface water extractions against extraction limits

Reasonable excuse

Frequently asked questions about the reasonable excuse claim.

Aerial view of Murray Darling Junction with flood waters flowing in near Lock 10. Location Wentworth

Frequently asked questions

The Murray-Darling Basin Authority (MDBA) sets a limit on the volume of water that can be extracted from an area (being a surface water valley, or a groundwater area). The Sustainable Diversion Limit (SDL) is the long-term average of how much water can be extracted out of each area. Under the Basin Plan each state is allowed to submit a reasonable excuse for why they exceeded this limit. We have submitted a claim for the Barwon-Darling as usage has exceeded the compliance trigger for 2020/21.

In 2019/20, the compliance functions for assessing a reasonable excuse claim sat with the MDBA. These functions have now been transferred to the Inspector-General of Water Compliance which was formally established on 5 August 2021.


NSW worked collaboratively with both the MDBA and Office of Water Compliance to finalise the reasonable excuse claim for 2020/21, including provision of supporting material to assist with understanding the rationale for the claim.

Download the report (PDF. 308KB)

How is SDL compliance assessed?

In the Barwon-Darling, and other surface water units, the permitted take is varied each year to reflect seasonal conditions. It does this by using models to estimate expected variability and to calculate the annual permitted take (APT). This is then compared to actual water use in that year, or the annual actual take (AAT). However, compliance under the Basin Plan framework is a cumulative compliance method. The yearly permitted and actual takes are added to the previous year's totals and for compliance the cumulative permitted take is compared with the cumulative actual take.

What was the exceedance?

NSW has completed the 2020/21 annual assessment of compliance with the SDL in the Barwon-Darling. The cumulative annual actual take (AAT) for 2019 – 21 exceeds the cumulative annual permitted take (APT) resulting in a balance of -68.8 GL. After adjustment for incomplete water recovery in 19/20 the balance is -66.9 GL. This debit is larger than the cumulative compliance trigger of -35.2GL.

The Inspector-General of Water Compliance will review the reasonable excuse claim. If the claim is granted, the Barwon-Darling with be compliant with reasonable excuse. If not, it will be non-compliant.

What was our reasonable excuse?

New meters were rolled out across the Barwon-Darling system over the last 10 years. The new meters are reported to generally read higher than previous meters over equivalent pumping events. The model used in the APT method was calibrated to match the figures returned by the previous meters. This results in inconsistencies between the current recorded diversion volumes and what is reflected in the model.

A claim has been submitted for the Barwon-Darling which demonstrates that errors in the metered data, and subsequently in the model used to assess compliance, is the main reason for the non-compliant outcome in 2020/21. This is largely a technical problem rather than evidence of growth in use. In addition, the Commonwealth still has not completed 1.6 GL of the planned recovery.

Why wasn’t the reasonable excuse accepted in 2019-20?

The department hadn’t implemented all of the procedures it said it would in the proposed water resource plan including the NSW extraction limit assessment. This work was completed in 2021 and concluded that current long-term average annual extractions are compliant with the Long-term Average Annual Extraction Limit (LTAAEL). Read our assessment (PDF, 165.17 KB) to find out more.

For background information on how these limits are defined and what the differences are between LTAAEL and SDL compliance, see our website.

What is being done to ensure compliance going forward?

The department submitted a ‘make-good’ plan to the MDBA in 2021 on how it will rectify the non-compliance. MDBA accepted these and also added some additional items in a published workplan. NSW completed all tasks that were required before the 2020/21 SDL compliance submission. The key tasks for ensuring compliance are described below. The meter recalibration project is the main outstanding task for ensuring ongoing compliance.

In addition to the tasks described below, NSW have agreed to a series of other make good tasks which reflect good water management practice. None of the additional tasks have any relevance for achieving a zero cumulative balance in the Barwon-Darling system.


Relevance to achieving SDL compliance zero cumulative balance

Remove historic embargo behaviour and inclusion of current restrictions in Barwon-Darling model

Correct representation of water sharing plan rules is essential for estimating APT

Metering Recalibration project adjustment of Barwon-Darling models project
In progress (Due end 2023)

Accurate representation of extractions that occurred under Cap conditions is essential for estimating APT

Accreditation of Barwon-Darling WRP
In progress (Submission due End June 2022)

No substantive changes to the method are planned beyond those described in the 2019/2020 and 2020/2021 reasonable excuse reports.

Regulation of floodplain harvesting
Measurement of floodplain harvesting take
In progress (Due end 2022)

The latest modelling for floodplain harvesting indicates growth in the Barwon-Darling is relatively small compared to the influence of meter recalibration.

What is the metering recalibration project and what will the outcomes be?

In the Barwon-Darling, the SDL and the water sharing plan limit (LTAAEL) are linked to the Murray Darling Basin Cap on diversions (Schedule E of the Murray-Darling Basin Agreement). The LTAAEL is equal to the Cap and the SDL is equal to the Cap minus the required water recovery.

The Cap Limit, Barwon-Darling Water Sharing plan limits and entitlement, as well as the underlying hydrological modelling, are based on historical extraction data that was derived from a ten-year period between 1995/96–2004/05.

The meters installed at that time did not measure extraction volumes directly. Rather, they recorded the time when a pump was operating and the average revolutions per minute of the pump. These are known as time and event (T&E) meters. The pump volume was obtained from a conversion factor applied to the T&E records. The conversion factors were known to not be 100% accurate.

The new meters return different volume readings than the old meters for the same volume of water pumped, with the error amount varying on each farm. It was recognised that revisions to the conversion factors would not only alter the historical extraction volumes but would also alter the modelling upon which the Cap (and subsequently the WSP and Basin Plan baseline diversion limit) were based. As reflected in the 2006 Heads of Agreement, the NSW Government and various stakeholders committed to revise the Cap volume based on better conversion factors once they became available. This update will also mean the SDL is updated for the Barwon-Darling as well as the method to calculate the annual permitted take (APT).

The proposed key steps are:

  • compare water take under old (time and event) and the newer pattern-approved flow meters to calculate a ratio for each pump site
  • use these ratios to recalibrate the historical take data on which extraction limits are based
  • recalibrate all the relevant models to the corrected historic data
  • identify what value the current extraction limits represent on the new (recalibrated) scale
  • amend the Barwon-Darling water sharing and water resource plans with updated extraction limit values based on the new, more accurate, scale. This includes the Barwon-Darling Long-term Average Annual Extraction Limit, the BDL and, consequently, the Sustainable Diversion Limit (SDL).
  • Where required, re-issue entitlements in volumes based on the new information and amend the water sharing and water resource plans.

Further information on the project can be found in our recalibrating Barwon-Darling water models using more accurate water metering records fact sheet (PDF, 188.9 KB).

When will the Barwon-Darling water resource plan (WRP) be finalised?

We are on track to submit the Barwon-Darling WRP for accreditation before end June 2022.

What does this mean for Barwon-Darling water users?

The department’s commitments to the Inspector General in the make-good plan will not negatively affect Barwon-Darling water user water allocation determinations or current practices in accordance with the water sharing plan. Actions in the make-good plan include improving metering calibration to better represent take, and continuing to assess compliance with the long-term annual average extraction limit.

The department will continue to engage with water users and other stakeholders on Basin Plan SDL compliance.