Anglers have reported hundreds of fish gasping for air on the surface of the Hunter’s "trophy bass dam".
Glenbawn Dam, near Scone, tipped 100 per cent capacity this week, which was when competitive angler Georgina Saunders said she noticed the "devastating" sight of dead bass.
"There was bass in different stages of rigor mortis, there was bass still dying, suspended down low laying on rocks, gasping," she said.
The Department of Primary Industry and Fisheries said it was aware of "a fish death event" at Glenbawn Dam and water quality testing had started.
The department estimated up to 1,000 Australian bass had been impacted, but Ms Saunders said it could be more.
"Glenbawn being such a massive waterway, to have dead fish floating along in every bay, that screams thousands," she said.
Ms Saunders said it was the topic of conversation among anglers who believed a lack of oxygen was to blame.
"There were a number of theories ... the cold snap and possibly all the growth on the side of the dam," she said.
Carp live on
Ms Saunders said plenty of people were still hooking carp.
"They seemed to be fine," she said.
"I've heard of it happening in other areas, other dams but it's generally been due to drought.
"I'm baffled."
Ms Saunders estimated some fish could have been up to 20 years old, and said they were longer than 35 centimetres.
"It's a trophy bass dam ... people flock to the dam's catch and release competitions."
The department said it would continue to monitor the situation with the assistance of WaterNSW.
