A crossbench Senator has accused the Federal Government of "undue secrecy" over its decision to withhold the legal advice that proves the Murray-Darling Basin Plan (MDBP) is lawful.

Key points: A Royal Commission showed parts of the MDBP may be unlawful

A Royal Commission showed parts of the MDBP may be unlawful The Federal Government has refused to release the advice which shows the plan is lawful

The Federal Government has refused to release the advice which shows the plan is lawful Water Minister David Littleproud said it was not common practice to release the advice

Taxpayers are halfway through funding Australia's most expensive environmental project — the implementation of the basin plan is expected to cost $13 billion.

The South Australian Murray-Darling Basin Royal Commission report — released at the end of last month — found parts of the plan and its management were unlawful.

Centre Alliance Senator Rex Patrick said the Morrison Government has an obligation to voters to release the expert legal opinion it is relying on to dispute that finding.

"This is not supposed to be cloak and dagger stuff," Senator Patrick said.

The Federal Water Minster David Littleproud has rejected the Senator's demand.

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

Senator Patrick said the Government's decision to withhold the information reinforced the commissioner's claim of "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 South Australian report found parts of the plan were unlawful. ( ABC News )

"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 told Senate Estimates during the week that releasing 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.

In the last sitting fortnight Senator Patrick, who represents South Australia, introduced a bill to ban cotton exports.