Greens senator Sarah Hanson-Young continued her attack on the coalition'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.

Centre Alliance senator Rex Patrick supports a royal commission, especially as the company involved in the $80 million purchase, Eastern Australia Agriculture, domiciles in the Cayman Islands.

"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.

He said the company made a $52 million profit in the transaction, money that went to the Cayman Islands.

"I actually think that the only way to get to the bottom of this is to have a federal royal commission," the senator said.

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."

Australian Associated Press