A report from The Australia Institute rejects government claims new dams are not being built, saying at least 20 to 30 large private dams have been constructed in the Murray-Darling basin in recent years.

While information on the number of private dams and the cost of their taxpayer subsidy is limited, the report says "it appears that just two of these dams cost taxpayers nearly $30 million".

"Over $200 million was spent on dam-related projects [in the Murray-Darling Basin] according to official data, although not all of this will have been specifically on dams," it says,

Maryanne Slattery, senior water researcher at the institute, said politicians don't want to talk about these dams because "they do nothing for drought-stricken communities, the health of the river or struggling farmers".

"These dams have been built on private land and are for the exclusive use of corporate agribusiness, such as Webster Limited," she said.

"Politicians are reluctant to talk about why millions of taxpayer dollars have been spent subsidising dams that make the problems of the Murray Darling Basin worse".

Water Minister David Littleproud has repeatedly berated the states for not building new dams. He said recently that of the 20 dams completed since 2003, 16 were in Tasmania.

"If NSW, Queensland and Victoria don't start building dams, their water storage capacity will fall by more than 30% by 2030," he said. "We put $1.3 billion on the table in through the national water infrastructure development fund in 2015 and have still had to drag most states kicking and screaming to build new dams."

The report says new public dams would require public consultation, including with stakeholders who had environmental and economic concerns.

But private dams involved "minimal public consultation and can be approved and constructed based on environmental assessments commissioned from private consultants by dam proponents".

The report looked at three dams in detail, on properties in the Murrumbidgee Valley owned by Webster Ltd - Glenmea, Bringagee and Kooba Station. The dams were funded out of the federal government's $4 billion water efficiency program.

The report argues such dams are not the best way to save water. It points to the department of agriculture and water resources saying new dams can save water where they replace shallower ones (which have more evaporation), or where they collect recycled irrigation water.

"However, none of the three case-study dams in this report save water in this way. They are new dams, not replacing smaller, shallower dams. Water stored behind their approximately eight metre high walls would otherwise be stored in public headwater dams around 100 metres deep."

These dams are designed to divert normal irrigation water and "supplementary water" - not to simply recycle irrigation water, the report says. Thus "they increase both evaporation and irrigation water use".

Supplementary water is water that is surplus to consumptive needs. It is important environmentally and to downstream users, historically making up almost all the water flowing from the Murrumbidgee into the Murray, the report says.

"With major dams now targeting this water, the Murrumbidgee could be disconnected from the Murray in most years. This has implications for all NSW Basin water users, who are already grappling with how to meet downstream obligations within the Murray's constraints and with no water coming down the Darling."

The report says a Canadian pension fund had just been reported as "swooping" on Webster, "with specific mention of a property with one of these new dams".

"The new dams that Australian taxpayers helped build appear to be highly valued by international investors," the report says.

Author: Michelle Grattan - Professorial Fellow, University of Canberra