Politics, not science dictated how the Murray-Darling Basin plan was implemented, leading to a flouting of the law and irrigators receiving too much water, the senior counsel to the Murray-Darling Basin royal commission has said.

In an interview defending the commission against claims from irrigators and critics who have questioned the experts the commission relied on to make its findings, Richard Beasley said the conclusions were clear and that meddling had “stuffed the river system”.

The NSW water minister, Niall Blair, said in response to the commission findings, “The NSW government would not have signed up to a basin plan that did not consider the social and economic impacts on communities … the suggestion by Mr Walker that further water buybacks should occur is strongly opposed by NSW as it would decimate our regional communities.”

The Labor leader Bill Shorten joined the defence of the commission by calling the treatment of the Murray-Darling “an ecological disaster” that was the responsibility of the Morrison government.

Speaking after Bret Walker’s final report accused the water authority of gross maladministration, Beasley told ABC radio: “We had witnesses come to the royal commission who work for the [Murray-Darling Basin Authority] who had previously said to the Australian people that the environment needs 4,000 gigalitres of extra water per year, up to 7,000 gigalitres of extra water per year on average.

“In less than a year, they changed that figure to 2,750 [gigalitres]. The reason that happened, witnesses told the commission – and it became a running joke in the basin authority – was that the figure just has to start with a two.

“In other words, it has to be two thousand and something or it is just not going to get done.

“That is politics, that is not science. And the amount of water for the environment had to be set by the best available science … and it wasn’t. That is why he [Bret Walker] has found it has been maladministration.

“… It is really important [to] Australians in a civilised country that the law is obeyed. It is important that happens. If the politicians don’t like the law, they can change the Water Act. But the Water Act is really clear. It says it is a statutory fact that we have stuffed this river system by giving too much water to irrigators and we have to stop that.”

Beasley said the report made it clear that climate change had been ignored in determining how much water the environment needed to keep the river system healthy.

“In 2009 – this is what the commissioner found to be negligent, not just indefensible, I think he uses the term grossly negligent – in 2009, the CSIRO completed some extensive work on climate change projections for the basin, funded by the government of the day,” he said.

“They then advised the MDBA, when you set the amount of water the environment needs to have recovered for it, you must, you must, factor in climate change projections.

“Originally the basin authority, in a draft of the basin plan, did that to a small extent. And that is where they came up with the figure of the 4,000 to 7,000 gigalitre range.

“They then totally ignored climate change for the purposes of setting the sustainable diversion limit.”

Beasley said no experts were turned away from the commission and the final report was built upon “a comprehensive body of scientific evidence”.

Neither the government nor the opposition have released an official response to the 746-page report commissioner Walker handed down on Thursday, but Bill Shorten pointed the finger at the federal government.

“This is an ecological disaster for the Murray-Darling and ultimately the buck stops with the federal government,” he said on Friday.

“They have been in power for the best part of six years and the conditions have got worse, not better.

“For a lot of Australians who don’t immediately visit the Murray-Darling Basin area they mightn’t appreciate how important it is to Australia’s biggest river system but the images of fish kills this summer have shocked people.”

Scott Morrison said it would take a bipartisan response to solve.

“The Murray-Darling Basin plan is a bipartisan initiative worked together through the parliament with support from ministers on both sides of the house and it’s important that as we continue to manage what is a very difficult and sensitive and complex issue, that we maintain the bipartisanship on how we progress that matter,” he said.

The damning findings in the report have led to calls from conservation groups for the Murray-Darling Basin Authority board to step down, with the Nature Conservation Council CEO the latest to call on the government to replace those on the board immediately.

“The position of the board and senior officials of the MDBA are now untenable,” Kate Smolski said.

“We call on the federal government to conduct a root-and-branch reform of the MDBA, starting with the removal of CEO Phillip Glyde and the board using a thoroughly transparent selection process to ensure the organization is not capture by special interests.”