The first fish die-off at Menindee killed perhaps tens of thousands of fish, while a separate incident three weeks ago left up to an estimated one million dead. This week's event, though, which began unfolding on Monday morning, could top the previous two. "The total biomass [of the current die-off] will probably be bigger," Mr Blair said.
While the bulk of the Menindee deaths were small bony herring, golden and silver perch were killed too, and some of the Murray Cod - which can be decades old and more than a metre long - are also turning up dead, Graeme McCrabb, a Menindee resident said, after meeting Mr Blair.
The trigger for the latest Darling die-off appears to be the sudden drop in temperatures in recent days after the mercury nudged 49 degrees last Friday. The shock of cooling to as low as 17 degrees triggered blue-green algae to die, bringing oxygen levels below critical thresholds for the fish.
Mr Blair said the fish deaths on the Murrumbidgee appear "to be the result of the same weather event".
The Department of Primary Industries said state and commonwealth agencies were meeting this afternoon "to discuss available options for managing flows within the Murrumbidgee River to address declining water quality at various locations".
Mr Blair said that unlike the Darling, there are more potential sources of water to help flush out a build-up of blue-green algae on the Murrumbidgee. These include Burrinjuck Dam, where the reservoir level sits at just under 38 per cent full, and Blowering Dam now at about 31 per cent, according to WaterNSW.
Flows at Balranald were about 500 million litres a day, a much better position for the river compared with the lower Darling, he said.
While Premier Gladys Berejiklian drew criticism for failing to travel far from Wentworth - at the juncture of the Murray and Darling rivers - during her recent visit, Mr Blair's second visit to Menindee was more positively received.
Mr McCrabb, who went out on a boat with Mr Blair on Tuesday, said a subsequent meeting between the Minister and other locals was "constructive", including the pressing of an economic recovery plan for the region. "He gave us the impression he wanted to help," Mr McCrabb said.
The problem could still get worse, with fish dying in parts of the Darling further out from Menindee than in the two previous kills, he said.
"The water looks terrible," Mr McCrabb said, adding the dead bony herring "are everywhere".
Menindee residents, meanwhile, are being asked to sign waivers for potable water deliveries, a move Roy Butler, the Shooters, Fishers and Farmers Party candidate for the seat of Barwon, said was to prevent them suing the government if they become ill.
"This is third world stuff. Sydneysiders would never tolerate that," Mr Butler said.
"The tap water is disgusting," he said. "I even met people who don't want to shower because it makes their skin peel."
Robert Borsak, the party's leader, said he would "be pushing very strongly for a NSW state royal commission when parliament resumes, to flush out the corruption and mismanagement of our rivers".