The Federal Government has been accused of “undue secrecy” over legal advice about the Murray-Darling Basin Plan (MDBP).

The South Australian Murray-Darling Basin Royal Commission found parts of the plan and its management were unlawful, but the Federal Government has not released the expert legal opinion it is relying on to dispute that finding.

“This is not supposed to be cloak and dagger stuff,” Centre Alliance Senator Rex Patrick said this week.

Federal Water Minster David Littleproud rejected the demand.

“As a general rule, [the] Government doesn't release legal advice on matters such as this,” a spokesperson from the Minister's office said.

Senator Patrick said withholding the information reinforces the “undue secrecy” around the plan.

“Royal commissioner Bret Walker, a most eminent barrister and SC, laid out in detail in his report why he believes the plan is unlawful,” Senator Patrick said.

“The basis of his claim is that the plan was not based on the best available scientific knowledge and was done with total disregard for the principle of 'environmentally sustainable levels of take' and was therefore developed contrary to the requirements of the Commonwealth Water Act of 2007.”

Senator Patrick said the Government believes the advice could prejudice future legal proceedings.

“If the Government has some legal 'knockout punch' why not put it out there so that costly proceedings are not initiated in the first place,” he said.

Senator Patrick has also introduced a bill to ban cotton exports.