The Greens are writing to Bill Shorten to back a royal commission into the Murray-Darling Basin plan if the Labor leader becomes prime minister at the May 18 federal election.

Greens senator Sarah Hanson-Young continued her attack on the Coaltiion's handling of the basin plan and in particular an $80 million water purchase made in 2017, the largest on record.

"This is just the latest in a long line of scandals and shonkiness in relation to the way the Murray-Darling Basin has been managed," she told Sky News on Sunday.

AAP

Prime Minister Scott Morrison had dismissed the need for further investigations into the water purchase on Saturday, saying there had already been a Senate inquiry into the matter where documents were produced.

But Senator Hanson-Young was unimpressed, saying those documents were heavily redacted.

"It's a bit cute to say the information has been given to the Senate when large parts of it have been kept secret, hidden from the public, from the taxpayer," she said.

"This is just more and more weight to the need for a royal commission."

Read More Government says no need for water royal commission

Centre Alliance senator Rex Patrick is on board for a federal inquiry, concerned the company involved in the $80 million water buyback resides in the Cayman Island.

"I am absolutely disturbed that the Australian government has been dealing with a company that is domiciled in a tax haven," Senator Patrick told ABC news.

The senator could hold a key role in the upper house crossbench after the May 18 election.