Authorities have taken river water sample in Walgett after "thousands" of fish turned up dead in Walgett waterways over the last few days.

Walgett resident Arthur Murray said he saw European Carp, Yellow Belly and Boney Bream when he entered the Namoi River on his boat on Monday 9 December.

There were less when the Aboriginal Medical Service transport driver went out the next day with Coonamble's Rob Ryan.

"In the 24-hour period, a lot of them had washed away or the pelicans ate them. There's hundreds of pelicans, and hawks and cormorants," Rob said on Wednesday 11 November.

"You'd smell the rafts (on Tuesday) of dead fish before you got to them. It's pretty pungent."

"It affects everyone because we can't swim in it or go fishing," said Arthur, who also said he saw thousands of dead fish on Monday.

Fish washed up on the river in town and could be smelled by residents. IMAGE: supplied

The Western Plains App understands the NSW Environmental Protection Agency has had personnel in Walgett this week taking sample.

The Barwon River level peaked 10.79 metres at around 6:45am on Tuesday 11 December in Walgett and is expected to fall below the minor flood level of 10.5 metres during Thursday into Friday.

While authorities have not confirmed a cause for the kills, they can occur when river height rises quickly. 

When a lot of organic material washes into the river, its rapid decay can consume dissolved oxygen from the water.

That can be a deadly mix with Summer, where higher water temperatures also reduce oxygen levels.