The Coalition is facing calls for an inquiry into the Murray-Darling Basin plan water contracts signed off by the former agriculture minister Barnaby Joyce.

As the Greens senator Sarah Hanson-Young called for a royal commission on Saturday, Bill Shorten also weighed in, saying there were now “question marks about the probity” of the “nation’s biggest water purchase”.

While the government dismissed calls for an inquiry, Shorten called on the prime minister to say whether he backed Joyce’s handling of the contracts and whether he would accept an audit.

“Produce all the documents, all the documents,” Shorten said. “… Is [Scott Morrison] going to stake his reputation on whether or not all of these matters have been done above board?”

Joyce, now a backbencher, signed off on the $200m in water buybacks in 2017. The process took place without an open tender and there has been criticism of the reliability of the water purchased, although the department says it undertook “due diligence activities”.

Guardian Australia reported this week on the mystery surrounding the reason Joyce chose the companies he did for the buybacks. On Friday, Labor raised specific questions about one of the purchases, worth $80m.

On Saturday, Hanson-Young said she would write to the auditor general requesting an urgent audit.

“The auditor has a responsibility to investigate how $80m of taxpayers money was paid for water that doesn’t exist,” she said in a tweet.

I am writing to the Auditor General today requesting an urgent audit into all water purchases signed off by Barnaby Joyce. The Auditor has a responsibility to investigate how $80m of tax payers money was paid for water that doesn’t exist. We need a Royal Commission. #watergate — Sarah Hanson-Young💚 (@sarahinthesen8) April 19, 2019

Morrison accused Shorten of “throwing mud around” during the election campaign. The prime mininster said the issue had been “raised before and has been addressed”.

“I understand the Senate inquired into the matter and sought production of documents from the government, regarding those transactions, which the government has provided,” he said. “So, that strikes me there is a high level of transparency.”

Simon Birmingham, the Liberals’ campaign spokesman, said a review had already been legislated under the Murray-Darling Basin plan.

“So that review will take place next year as is already required,” he told ABC television.

The $80m purchase in question is reportedly the highest ever, but Birmingham said the drought had pushed prices up.

“It is not a surprise in the last couple of years you would be paying more for water licences in a condition where the market has tightened and the availability of water is less,” he said.

One of the companies to benefit from the water contracts was EAA, of which the current energy minister, Angus Taylor, was once a director. Taylor’s office said he “never had a direct or indirect financial interest in EAA or any associated company”.

“Taylor concluded all association with EAA and related companies prior to entering the parliament,” a statement said.

In a statement released on Friday, the agriculture department said the purchase “represented a unique opportunity to secure a significant volume of water in a catchment of particular strategic importance”, and that “claims that the water cannot be used off the property are false”.

“Water entitlements are now held by the Commonwealth Environmental Water Holder,” the department said.

“EAA has decommissioned structures that previously allowed it to capture the water. This water is no longer being used for irrigation or being obstructed by on farm infrastructure thus allowing the water to flow across the land into waterways. It has in-stream environmental values helping to improve the health of the river while being available for use to meet environmental targets downstream.”

The department added that “negotiations on the purchase were conducted directly with Eastern Australian Agriculture” and that the purchase was “consistent with Commonwealth Procurement Rules and paid at a fair market rate”.

“The department undertook due diligence activities in investigating the proposal, including checking the validity of the licences on offer; and obtaining commercial water valuation advice, independent advice on the possible socio-economic impacts, and advice from the State Government, the MDBA and the Commonwealth Environmental Water Holder.”