The South Australian royal commission into the Murray-Darling basin plan looks set to wrap up without hearing from any federal bureaucrats responsible for the plan after the SA government said it would not extend the reporting deadline.

There are now urgent efforts being made by crossbench senators in Canberra to use Senate powers to demand crucial documents sought by the commission. But even this manoeuvre may not be successful if the Turnbull government decides to play hardball.

The findings of the royal commission would be considerably weakened if it does not hear from officers from the Murray-Darling Basin Authority or at least obtain key documents it holds.

Earlier this year, the federal government barred MDBA staff and other federal officials from giving evidence or producing documents and sought an injunction in the high court to prevent them being subpoenaed.

The high court case, which turns on the constitutional question of whether a state royal commission has power to demand evidence from federal bureaucrats, is due to be heard in October. A decision of the full high court could take months.

The royal commission has a reporting date of 1 February.

On Friday the commissioner, Bret Walker SC, wrote to the SA attorney general, Vickie Chapman, explaining that he did not feel he could provide procedural fairness to federal witnesses if he had to report in February. He said it may be necessary, as a model litigant, to withdraw from the high court litigation. He said an extension on the reporting date was a matter for the government.

The SA Liberal government then issued a press release saying he had not sought an extension – which the commissioner disputed – and that the commission would wrap up in February.

After an angry exchange of letters and press releases, Chapman confirmed that there would be no more money and no extension.

The events highlight how politicised the Murray-Darling basin plan has become, even as the river system struggles to deal with the latest drought conditions.

The SA inquiry had been called by the former Labor SA government on the eve of the state election amid an acrimonious dispute between the state governments over amendments to the basin plan that would cut the amount of water recovered for the environment.

The commission has received only lukewarm backing from the new SA Liberal government and was directly opposed by the Turnbull government as unnecessary.

The commission has been zeroing in on how the MDBA came up with the 2,750GL environmental water recovery target under the plan, and whether it complied with the underlying legislation, amid evidence from former MDBA staff and scientists that the target had little scientific backing and was a political compromise.

If the original target for recovering water for the environment is flawed, it would have direct impacts on the latest amendments to the plan which seek to further cut the targets for recovering water and instead use a suite of projects to achieve equivalent environmental outcomes.

These are highly controversial, particularly for SA at the end of the river system. The commission has heard evidence that the projects lack scientific rigour and there is no certainty they will deliver the environmental outcomes promised.

Whether the inquiry can gain access to federal documents via the Senate is unclear.

The Centre Alliance’s environment spokesman, Senator Rex Patrick, has said his party would use its crossbench weight to ensure the commissioner was supported in his inquiry.

“The commissioner has been foul snookered and that’s not appropriate,” Patrick said.

Senate notices to produce documents are usually carried in the Senate without a formal vote but the Turnbull government could insist on a formal vote, putting Labor under pressure on its position. The original basin plan was devised by the Labor government.

It is also unclear whether the SA commission could use the documents as evidence once they are in the public domain.

