Legislation designed to ensure the Murray-Darling Basin is managed in the national interest excludes structures operated by the Snowy Hydro scheme, meaning Tantangara Dam can capture more than 90 per cent of annual flows of the Murrumbidgee.

Excessive capture upstream has the potential to damage river ecology and water security in and around Canberra, a group campaigning to have Snowy Hydro assets included under the Water Act say.

Campaign manager Andy Lowes said a lot had been learned about better river management since the Snowy Water Inquiry Outcomes Implementation Deed, which limits the harvesting of water from the Snowy and Murray Rivers, was drawn up 20 years ago.

The Deed did not place any limits on harvesting of Murrumbidgee River water.

A former employee of the Murray Darling Basin Authority and Commonwealth Environmental Water Office, Mr Lowes directed his attention to the Murrumbidgee during the 2019 drought, when water had to be trucked into Tharwa, 30 kilometres from Canberra.

Having grown up in Tuggeranong, Mr Lowes said he watched as the Murrumbidgee dried up and as water was supplied to protect the town from fires raging in nearby Namadgi Nation Park.

"I thought, 'that's not really supposed to happen'," Mr Lowes said.

"It made me start investigating my own river a bit more."

The Australian River Restoration Centre want downstream water users, as well as cultural and social impacts along the Murrumbidgee, considered in Snowy Hydro's future operations, in addition to its energy production and distribution goals.

The group have sought a commitment from Snowy shareholders, Finance Minister Katy Gallagher and Energy Minister Chris Bowen, to review the rules governing Snowy Hydro's water use.

In a letter to the ACT senator, the not-for-profit organisation detailed the inconsistency of the Murray-Darling Basin Plan and the Snowy License having statutory review periods of 5-10 years, whereas the Deed governing Snowy Hydro operations has none.

ACT Minister for Water Shane Rattenbury as included in the communication with Ms Gallagher.

He said he'd raised the suggestion of reviewing the operation of the Snowy Hydro Scheme and the Snowy Water Outcomes Implementation Deed with Environment Minister Tanya Plibersek.

Mr Rattenbury said conducting federal reviews would highlight changes required to the Water Act and the Basin Plan to establish appropriate governance arrangements to ensure sustainable operation of the Snowy Hydro Scheme.

"The Upper Murrumbidgee provides critical ecological and cultural functions and water supply for populations in both the ACT and NSW," Mr Rattenbury said.

"However, up to 97 per cent of the natural river flow is diverted from the Murrumbidgee River by the Snowy Hydro Scheme for hydropower generation.

"The reduced flow, in combination with climate change and upstream water management arrangements, have impacted on Traditional Custodians cultural values, aquatic wildlife and water security for critical human water needs.

"The current operation of the Snowy Hydro Scheme does not create a sustainable future for the Upper Murrumbidgee Region."

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Change in river hydrology and the environmental impacts associated with Snowy Hydro diverting more than 90 per cent of river flow was reported by the ACT Commissioner of Sustainability and the Environment in 2019, a government spokesperson said.

"Scientific reports highlight significant decline in native fish populations, sedimentation of the Murrumbidgee River channel, and poor water quality entering the ACT," they said.

With the Murrumbidgee a third water source for the ACT, behind the Cotter and Googong, Mr Lowes said another year or two of drought in 2019 could have impacted water security in Canberra.

Mr Lowes said he still can't understand why the Upper Murrumbidgee wasn't included in the Water Act. He suggests it could be that rules were already in place and they didn't want to duplicate or impact existing legislative framework.

"I went from working in these government agencies and going out to others to explain how the Murray River works and now I find myself as a community member, trying to work out how our reserve works," he said.

With Ms Plibersek recently declaring nothing was off the table in the bid to return 450 gigalitres of environmental flows for the Murray-Darling, ahead of the 2023 deadline, Mr Lowes said perhaps the Snowy could play a part.

"The problem is they're trying to find 450 gigalitres in an over-allocated system, but nobody's had a look at the Snowy Scheme yet," he said.

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