Residents in Narromine Shire say at least 150 fish have turned up dead in the Goan Waterhole in Trangie.

Local grazier Paul 'Camo' Cameron said he first become aware of the kill on Sunday 19 October.

He said they were mostly European Carp, although also native species like Cod.

"There's a lot of beautiful Yellowbelly (Golden Perch) there of all sizes," he said.

"There would have been one there that was about at least three or four pounds."

The Western Plains App understands NSW government authorities are aware of the issue.

Narromine-based Tony Lees is involved in environmental restoration work with the waterhole and said he observed over 200 fish in the kill.

Fish kills are more common in Summer. [IMAGE: Tony Lees]

"We've got some pretty decent flows coming down our river now but of course they don't get out of the river and follow those natural water courses anymore. The river's not high enough," he said

"The problem with the system is that without those natural flows we're not replacing that water.

"It's just stagnant, still water.

"The only top up it gets is town water, which is not great.

"Even though it's rain water, it's coming out of drains and off roads, so it's not good quality water coming in."

It is not yet confirmed what caused the kills.

However, Mr Lees said it is likely that increased temperatures killed floating duckweeds in the waterhole, which then sank below the surface and sucked oxygen from the water as they decayed.

Narromine Shire mayor Ewen Jones said there was a voluntary clean up of the weed around three years ago, which he attributes to probably saving "hundreds" of fish.

He also said yesterday (21 October) that locals told him they expect further fish deaths today.