The NSW government has warned there will likely be more fish kills in the coming days due to "high temperatures".
Primary Industries Minister Niall Blair says the mass death event at Menindee last week was caused by a "perfect storm" of factors including severely low water flow, algal blooms and a sudden drop in temperature.
The NSW minister says mismanagement wasn't the reason up to a million fish died in the Darling River system but there's more bad news to come.
"Due to high temperatures, we do expect to see more fish kills across parts of the far west and Northern Tablelands this week," Mr Blair said in a statement on Sunday.
"We are working with local councils, recreational fishing groups and community groups to both manage these events as well as providing assistance for clean-up operations."
Scientists say rotting fish need to be "urgently" removed from the Darling River or the carcasses will trigger even more deaths.
NSW Labor on the weekend accused the coalition of changing the rules of the Barwon-Darling Water Sharing Plan in 2012 despite being warned by fisheries experts that doing so could lead to insufficient environmental flows.
But Mr Blair hit back on Sunday saying the amendments proposed by Fisheries NSW were included in the 10-year plan and remain in place.
The so-called "review clause" - which aimed to secure environmental outcomes in low and medium flows - is in part 12 of the plan as the minister noted.
But qualifications were added by the government so that any change had to "take into account the socio-economic impacts of the proposed rules" and "not substantially alter the long-term average annual extractions".
Leading environmental scientist John Williams last week argued the entire Murray-Darling basin had been mismanaged by successive governments and there needed to be a 30 to 40 per cent reduction in the amount of water extracted by irrigators.
Federal Labor leader Bill Shorten wants an emergency task force to investigate the mass death event at Menindee.
"We need to methodically identify the precise causes of this disaster and then act on them immediately," Mr Shorten has written in a letter to the prime minister.
NSW Opposition Leader Michael Daley has promised a Labor state government would establish a special commission of inquiry into water management with royal commission-like powers.
Australian Associated Press