Graeme McCrabb, a resident in the town of Menindee, said on Monday morning that he could see the dead fish - mostly bony herring - stretched out along at least a kilometre.

"It's started again," Mr McCrabb said. "Any fish that's still there is struggling."

"It's a sick spot of the river," he added, saying he could also see other fish, such as carp, trying to find enough oxygen to survive. "They are dying in front of us."

The stretch of the river, known as the Menindee weir pool, was the site of a mass die-off that numbered as many as one million fish earlier this month. Before Christmas, there was another fish kill in the area, that also left thousands of fish dead.

The cull included many giant Murray cod, some decades old, as well as golden and silver perch, the latter of which is endangered.

That fish kill was blamed on low oxygen levels in the near-stagnant waters, with little or no flow coming into the river. The nearby Menindee Lakes were just 3.2 per cent full as of Sunday, according to WaterNSW.

The Menindee region has turned sharply cooler overnight, with temperatures down to about 18 degrees as of 8am on Monday. On Friday, the mercury peaked at a sizzling 48.8 degrees, with the two days following topping 45 degrees.

There was also a couple of millimetres of rain overnight in the region.

In the previous big fish kill, the drop in temperatures caused some of the blue-green algae to die, depleting already low dissolved oxygen levels further.

The overnight rain may also have mixed the stratified layers of water, reducing the ability of remaining fish to find pockets of relatively high oxygenated waters.

It's understood that fisheries experts from the Department of Primary Industries are headed to Menindee to examine the latest fish deaths.

The latest die-off was "precisely why we need an independent review into the circumstances in the Menindee Lakes," Chris Minns, Labor's water spokesman, said.

Mr Minns also called on the Berejiklian government and other jurisdictions in the Murray-Darling Basin to drop plans to decomission the Menindee Lakes system.

"The governments' plans to close down the Menindee system will see this disaster happen again and again, he said, adding that a stop to the project was "what the locals are begging for".

Jeremy Buckingham, the formerly Greens and now independent MP,  said "this ecological collapse is caused by human abuse of both our river systems and the climate".

“This additional fish kill could have be avoided if more water had been kept in storage in the Menindee Lakes system to flush the river," he said. "It only highlights the mismanagement of the river."

Premier Gladys Berejiklian said the problems facing the region were because of the "unprecedented" drought, adding he government "could not control the weather".

"These are very difficult drought conditions, exacerbated by a number of other factors," she said.

The Premier defended her decision to visit Wentworth on the Murray but not to tour Menindee, saying she was on the ground "in close proximity". The two towns are about 240 kilometres apart.

Her government's priority was to secure water security for far-western towns, and cited support for bores in towns such as Walgett, and a soon-to-be-opened pipeline from Wentworth to Broken Hill that Labor had opposed.

Niall Blair, the fisheries minister, was not available for comment on the latest fish kill.

His office did, though, provide comment on a recent rescue and successful relocation of several large Murray cod that had been struggling to survive in the Darling River near Menindee.

“[G]enerally, translocating fish is not a solution because of the additional stress it places on already stressed fish," Mr Blair said, adding that DPI staff were "prepared to do whatever they can to help our native fish".

The fish were relocated 15 metres upstream after an aeration period in tubs "to give them a fighting chance”, he said.