South Australia's royal commissioner investigating the Murray-Darling Basin plan has taken a swipe at the federal water resources minister over a letter sent to his state counterpart on issues forming part of the inquiry.
Commissioner Bret Walker has described the actions of David Littleproud as "irregular and deplorable" at the commission's public hearing in Adelaide on Wednesday.
At question is a letter and attachment Mr Littleproud sent to SA Environment Minister David Speirs, which rejects the need for the royal commission and backs the constitutional validity of both the basin plan and the federal Water Act.
Mr Walker said the letter was no way for the minister to communicate with the commission and was a "highly irregular and inappropriate" way for a submission to be presented.
There were also a number of "highly contestable propositions of fact, law and mixed fact and law" contained both in the letter and in the "so-called" submission, he said.
"It need hardly be said that one of the reasons why this is an irregular and deplorable way for the minister to have proceeded is that it prevents the very kind of engagement, confrontation, challenge, and refinement of legal and factual positions which would be permitted would submissions to be put and evidence to be tested," Mr Walker said.
In his letter, Mr Littleproud said the royal commission amounted to a fundamental review of the basin plan and its legislative underpinnings which the commonwealth considered unnecessary.
The minister also touched on the federal government's decision to take High Court action to question the power of the commission to force government employees to appear and give evidence and said the commonwealth believed the basis on which the basin plan was developed was consistent with the requirements of the Water Act.
"As a final point, I note that the royal commission has also raised questions about the constitutional validity of commonwealth legislation and has indicated that the validity of the Water Act will likely be the subject of a subsequent Issues Paper," Mr Littleproud wrote.
"The government considers that both the Water Act and the basin plan are constitutionally valid and will consider any subsequent issues paper on these issues carefully."
Despite his criticism, the minister said the commonwealth remained committed to working collaboratively and in partnership with basin states to ensure the successful implementation of the Murray plan in full and on time.