The Yanco Creek proposal aims to return 20 to 35 gigalitres to the environment to help reach the Murray Darling Basin Plan's 605GL water recovery target, by cutting of the natural flow of Yanco Creek and redirecting it along the Murrumbidgee.

But if it gets approved, it could be subject to a legal challenge because of its dodgy modelling.

Government documents show the original modelling didn't justify the project, so the water agencies behind the proposal "simply changed" it.

"This threatened the justification of the savings from the supply measure, because savings from 'transmission losses' were too low," the project's 2015 business case states.

"To overcome this, DPI and MDBA simply changed the benchmark model to increase the savings."

Water consultant Maryanne Slattery uncovered the dodgy modelling in a recent report and said it was a clear violation of the Murray Darling Basin Plan (MDBP).

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"The Basin Plan and the Water Act are clear in how these projects need to be measured and modelled, but these requirements are not met in the Yanco Creek System," Ms Slattery said.

The only time benchmark modelling can be changed under the MDBP is by "the addition of the notified supply measures" and "the removal of any unimplemented policy measures".

"There is no provision in the Basin Plan to amend the model in the event that the benchmark model contains errors or that the modelled results do not yield the results that were expected or required to justify a supply measure project," the report states.

"In other words, the original modelling did not estimate a water saving to the liking of the water authorities, so the modelling was changed to make it more acceptable. This is a breach of the method set out in the Basin Plan and could also be subject to a legal challenge."

The Land asked the Murray Darling Basin Authority (MDBA) why the project was allowed to proceed when the modelling was in breach of the MDBP, however it did not answer the question directly.

"All the issues raised need to be considered through the next stage of development of the project and resolved before the project moves to implementation and construction," a MDBA spokesperson said.

The two projects proposed for Yanco Creek are an $84m weir extension to stop the Mumbidgee River flowing in to Yanco Creek and on to the Murray River, and the rebadging of water to make it more accessible to downstream irrigators.

About 250 licence holders rely on the Yanco Creek system and have raised serious concern about their future access to water should the proposals go ahead.

The Australia Institute and Slattery and Johnson report found the water saved from Yanco Creek would be turned into additional water entitlements for downstream Murrumbidgee irrigators.

"This water will no longer flow down the Yanco Creek system [and eventually into the Murray], but down the main channel of the Murrumbidgee," the report states.

"It is likely to encounter difficulties there, particularly given the increase in irrigation activity in the lower Murrumbidgee.

"In other words, water will be taken out of the Yanco Creek system, potentially denied to some Murray users and diverted into an entitlement with uncertain reliability."

The Land contacted the NSW Department of Planning, Industry and Environment (DPIE), asking if it could guarantee the water saved would not be used by irrigators and be returned to the environment as intended, however it did not respond.

In January, DPIE said it was "continuing to explore options for the offtake regulator".

"The department is currently in discussions with a Community Advisory Group and Technical Advisory Group to discuss the different options and to co-design elements of the projects," DPIE said, at the time.