Labor leader Bill Shorten has urged Prime Minister Scott Morrison to establish an emergency task force to investigate one million dead fish in NSW.

The opposition leader wrote to the prime minister, calling on the federal government to act thoroughly and immediately to address mass fish deaths in the Darling River at Menindee.

"If this ecological disaster has been worsened by human activity, we must quickly get to the bottom of how it took place," Mr Shorten wrote.

The task force would investigate how and why the fish died, including the potential impact of agricultural chemicals.

Mr Shorten also wants it to look at whether water diversions or management in the Murray-Darling system exacerbated the scale of the disaster and recommend immediate steps to help the basin.

Labor would consider working with the government to urgently pass legislation through federal parliament to address the crisis if recommended by the task force.

It comes after NSW Labor lobbied the Berejiklian government to establish a special commission of inquiry to look at the issue.

"The immediate priority is to ensure we have the science before us to make decisions for a healthy river system," Mr Shorten wrote.

"If the task force's findings warrant further investigation, Labor will strongly consider what further inquiries need to take place, including a possible judicial inquiry."