Among documents that the agency handed over, the authority refused to disclose key valuation details about water buybacks from irrigators, some of which involve prices well in excess of independent assessments.

"Valuations are commercially sensitive until the property's water is sold," Senator Patrick said, adding they had "little meaning" a year after.

The authority declined to answer any of a list of questions sent by Fairfax Media.

Information was withheld because of a range of reasons, including their commercial sensitivity or the possibility of affecting federal-state relations, according to a letter earlier this year by resource minister Senator Matt Canavan.

'Protection racket'

“The government and the Murray Darling Basin Authority are favouring secrecy over transparency when it comes to their mismanagement of the Murray Darling Basin Plan," Sarah Hanson-Young, the Greens basin spokeswoman, said. "Their protection racket has got to be busted open."

"Banks don’t get to just ‘opt out’ of appearing before the Banking Royal Commission," Senator Hanson-Young said. "It doesn’t fly that the MDBA and the government are standing in the way of the truth coming out and moving forward on repairing the broken trust in river communities."

The management of the Murray-Darling Basin has come in for close scrutiny over the past year after an ABC Four Corners program made accusations of widespread water theft. Progress on securing water for the environment is also likely to be tested as much of the region endures drought conditions.

The Royal Commissioner Bret Walker SC is expected to mention the High Court injunction when public hearings begin on Monday, although he "will not take any active part in those proceedings", a commission spokeswoman said.

The authority could be in for an embarrassing few weeks, particularly if evidence is presented that showed its modelling was altered to address political rather than scientific concerns.

David Bell, an experienced water planner at the authority from March 2009 until his retirement as director on November 2017, will tell the Royal Commission that the $13 billion Basin Plan that was heavily influenced by the need to strike an agreement with "hostile states", vested interests and flawed science, according to an ABC report last week.

"I am concerned that, unchecked, the current Basin Plan implementation will lead to worse environmental outcome than would have arisen if there were no Basin Plan," Mr Bell said in his statement.

NSW policy

Water issues are also likely to feature in NSW this week with state parliament expected to debate its Water Management Amendment Bill 2018 this week.

The NSW Greens say the Berejiklian government has weakened its own draft bill, taking on board advice from irrigators as outlined in their submission.

“Some of the provisions in the bill currently before parliament are a recipe for killing the Darling River," Jeremy Buckingham, NSW Greens spokesman, said.

"Essentially you will have water extraction concentrated in the upper reaches, while the lower Darling dies and Broken Hill is fed by a pipeline from the Murray at Wentworth."