A historic fish kill in NSW's Darling-Baaka River will be treated as a "pollution incident" as the state's Environmental Protection Agency investigates the cause.
Millions of dead fish surfaced at Menindee in the far west of the state earlier this year, leading Premier Chris Minns and Environment Minister Penny Sharpe to visit the region.
Low levels of dissolved oxygen in the water, known as hypoxic blackwater, a condition made worse by prolonged floods and unseasonal heatwaves, was blamed for the deaths.
On Wednesday, it was revealed the NSW EPA will investigate potential causes of the mass fish kill and whether any pollution offences were committed that would require regulatory action.
Ms Sharpe said too many questions had gone unanswered as locals deal with the fallout of the historic incident.
"The community has told me that the response by all agencies to this event hasn't been up to scratch," Ms Sharpe said.
"I have seen first-hand First Nations and community connection to these waterways.
"I will leave no stone unturned to ensure we do what we can to prevent any further adverse environmental impacts."
The EPA has power under the Protection of the Environment Operations Act to probe individuals, companies and government entities over the incident.
This includes statutorily obtaining information and records, conducting interviews and collecting samples.
"No one is above the law - including the government," Ms Sharpe said.
"It is my expectation as minister that should the EPA uncover any breaches of the Act, they will enforce regulatory action."
Initial testing by the EPA revealed the affected area of river contained potentially toxic algae and blue-green algae.
Testing to date has ruled out a pesticide pollution event, with results coming back negative for more than 600 pesticides.
Further testing results are expected to be shared with the community as early next week.
