Authorities have confirmed high flows and climbing temperatures contributed to thousands of fish deaths in Walgett.

A spokesperson for the Department of Primary Industries and Regional Development said a kill affecting about 2000 fish, mainly Carp, was reported on Monday 10 December.

Fisheries Officers with the Dharriwaa River Rangers attended sites on 13 December and noted critically low levels of dissolved oxygen (DO) in sections of the lower Namoi and Barwon rivers. 

"The area has received recent high flows, followed by increasing temperatures, which has impacted water quality including dissolved oxygen and contributed to the fish kill," a Department spokesperson said.

"The NSW Government is monitoring conditions downstream in partnership with the Commonwealth."

In the Walgett, the Barwon River's height peaked 10.79 metres at around 6:45am on Tuesday 11 December, according to the Bureau of Meteorology.

An image of dead fish taken on Tuesday 11 December in Walgett waterways. IMAGE: supplied

The river in the town sat at 10.42 metres and falling slowly, below the minor flood level of 10.5 metres, according to the midday update on Friday 13 December.

Authorities also said there is no concern for Walgett's water supply.

"Walgett’s town drinking supply is currently sourced from bore water and is not impacted by river conditions or the fish deaths," a spokesperson for the NSW Department of Climate Change, Energy, the Environment and Water said.

"We always remain vigilant about prioritising water security for towns, critical human needs and the environment, which is why we are always monitoring raw water supply.  

"Once flows have receded at Collarenebri, further upstream, we will have a clearer idea about when river flows might return to normal for Walgett and other downstream locations."