"These fish are struggling."
Temperatures in Menindee climbed above 47C for three consecutive days this week and the mercury is forecast to remain in the 40s until Friday.
A cool change is expected on Saturday, when the temperature is set to drop to 36C.
That's when residents fear the same water conditions which led to last week's mass fish deaths will cause another event.
"A major concern is that once that temperature drops away this will happen again," Mr Gregory said.
Up to a million fish died in the river system early last week when a cool change swept through the region after a hot period.
Authorities believe the change may have disrupted an existing algal bloom, killing the algae and depleting oxygen which worsened water quality for fish.
NSW Primary Industries Minister Niall Blair is also concerned but hopes aerators installed on Wednesday will boost oxygen levels in the water and prevent further fish kills.
"The challenge will be later in the week when temperatures will drop again and we hope we don't see the algae dying off again," Mr Blair told 2GB radio on Thursday.
"It's not a silver bullet, we know its a bit of a bandaid solution, but we are going to try everything we can to make sure we don't see a repeat of what we saw out at Menindee nearly two weeks ago."
The aerators are part of a suite of strategies discussed at an emergency meeting with scientists and river managers, convened by the Murray-Darling Basin Authority on Tuesday, to help prevent further fish deaths
The NSW and federal governments have been under pressure by scientists who say mismanagement of water is to blame, and some are calling for the Murray-Darling Basin Plan to be overhauled.
Deputy Prime Minister Michael McCormack on Thursday said he is committed to the bipartisan water management plan, but added the fish kill meant there was cause to revisit it and improve it.
The Australian Greens are calling for a royal commission into the management of the river system after the deaths, pointing the finger at "corrupt" irrigators.