Poor oxygen levels have been blamed for a mass fish kill in Newcastle's Throsby Creek.
Thousands of small to medium-sized dead fish were floating in the main channel between Maitland Road, Tighes Hill and Hannell Street, Wickham during Monday morning's high tide.
Bream, whiting, mullet, catfish and eels were among the species seen in the water and strewn around the banks.
A growing stench of rotting fish hung over the waterway yesterday afternoon, particularly around the Carrington mangrove forest where many of the fish were trapped.
In addition to a potential toxic threat, there were also fears the dead fish may attract bull sharks into the shallows in coming days.
The dead fish started appearing on the weekend.
"I saw a few of them in the eddies near the stormwater outlets at about 10am on Sunday. As I kept walking there were more and more of them," Maryville resident Eddie O'Reilly said.
The creek's 3000 hectare highly urbanised catchment covers much of Newcastle and extends to the Charlestown ridgeline.
NSW Environment Protection Authority and Department of Primary Industries fisheries officers investigated the fish kill on Monday.
A spokesperson said there was no evidence of a polluting discharge from an industrial source.
"All indications suggest the fish kill was caused by critically low dissolved oxygen levels in the creek," he said.
"Further investigations identified that a build-up of leaf litter in the Newcastle area, followed by a short high-intensity rainfall event, along with increased temperatures, are likely to be contributors to the low oxygen levels and fish kill."
The incident was similar to blackwater events in the Hunter River in April last year and in 2013.
The creek's condition has also seen property prices around the main channel increase significantly.
"The creek has been going from strength to strength. This is a total shock. We need to get to the bottom of it," Tighes Hill resident Jonathon Love said on Monday.
Another local, Daryn Steer, said it appeared the fish kill had been caused by a sudden change in water quality.
"It is very rare to see a dead fish in there. There were a couple floating in there on Sunday but this morning was just insane," he said.
"The place is normally teeming with birdlife and fish. It looks like whatever has happened has come on very suddenly."
Numerous government agencies including the Department of Primary Industries, Environment Protection Authority and Hunter Water share responsibility for the waterway.
Hunter Water has responsibility for the Throsby Creek channel to the Hannell Street bridge and owns the stormwater channels upstream of that point.
A spokesman said staff had inspected the creek on Monday and confirmed there had been no operational issues in the catchment.
Newcastle MP Tim Crakanthorp, who chairs the Throsby Creek Government Agencies Committee, said he was deeply concerned about the incident.
"I am deeply concerned about the mass fish kill and extremely keen to see the reason behind it," he said.
