Another 80cm Murray cod has died in the Lower Darling River on Friday afternoon, raising fears among Menindee locals that the predicted drop in temperature overnight will trigger another fish kill incident.

The Guardian saw the large cod, estimated to be about 30 to 40 years old, floating about 1km downstream from where another large cod died on Thursday.

After three days of temperatures over 40C, the temperature is due to drop to 24C overnight. A temperature drop can lead to algal blooms dying or rapidly de-oxygenating river pools during the night as they go into a reverse photosynthesis.

The major fish kill on 7 January followed a heatwave and a sudden drop in temperature to 14C.

Locals are patrolling the river looking for signs of a repeat incident. The NSW Department of Primary Industries has kept a team of fish clean-up experts in Menindee in case of a new outbreak. They have been out before dawn looking for dead fish.

Earlier on Friday, the commissioner heading the South Australian royal commission into the Murray-Darling basin plan offered to extend his inquiry to encompass the fish kill at Menindee, after the New South Wales government appeared to blame SA for the draining of the lakes. The commissioner’s offer is understood to have been rejected by the SA attorney general, Vickie Chapman.

Bret Walker SC is due to hand his report to the SA government within days, and it is expected to deliver a scathing assessment of both the management of the plan by the Murray Darling Basin Authority and the underpinning water recovery targets that aim to restore the basin’s health.

But at this stage there is no indication from the SA government about when it will be released and and there are fears the Liberal government could seek to hold it up until after the NSW election.

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“It is my view that the whole report should be published immediately,” Walker said. “The public interest demands it. There are no countervailing considerations such as national security or the administration of justice.”

Scott Morrison has called for better information in the debate.

“I think we need to look carefully at what is actually occurring,” the prime minister said. “Of course, the drought, as the deputy prime minister has said, has had a devastating impact on what we’re seeing, and there has been a perfect storm of other environmental factors, which has crystallised into the serious fish death that we’ve seen.

“But before we start ripping up bipartisan agreements that have been very important to how we manage that area, I think it’s important that we inform ourselves more.”

Play Video 1:46 ‘It’s a depressing sight’: Another mass fish kill expected in Menindee – video

Behind the scenes there have been tense exchanges between the commission and the SA attorney general’s department.

The government says it will release Walker’s report on a departmental website and intends to shut down the royal commission website by 30 March.

Walker has argued this is out of step with normal procedure and would result in submissions and transcript to his inquiry no longer being available.

The NSW opposition has promised a recovery plan for the Menindee lakes and vowed to halt a plan to shrink the lakes. This is a key part of NSW’s strategy to meet its environmental water recovery target and would mean that water has to come from elsewhere else.