A spokesperson from the NSW Department of Primary Industries (NSW DPI) says the current conditions at Lake Cathie are likely to continue.
"Poor water quality in our coastal lakes is likely to continue, without significant rainfall," she said.
The statement comes after NSW DPI Fisheries staff attended the site on Wednesday, February 27 and observed up to 1000 dead fish along a section of Lake Cathie. The area was isolated from the main body of the lake.
The NSW DPI spokesperson said the main species were juvenile mullet and a smaller number of bream.
"The suspected cause is related to the very low water levels at Lake Cathie and the resulting isolation of pools," she said.
"These pools are then susceptible to high water temperatures and low dissolved oxygen."
The spokesperson said DPI Fisheries will continue to work with council on monitoring Lake Cathie.
Residents are shocked by the state of their local waterway saying they have not seen it in such a critical condition in decades.
Port Macquarie-Hastings Council has no plans of opening the lake despite the critically low water levels reaching the trigger point for an artificial opening.
An expert in water ecology says problems with the health of natural water systems tend to arise with development and human habitation.
Dr Paul Humphries is a professor at Charles Sturt University and based at Albury/Wodonga. His interests mostly relate to rivers and their ecology.
Most of his work over the last 20 years has focused on the relationship between riverine biota and flow.
While Dr Humphries couldn’t comment directly on the health of Lake Cathie’s water system, he anecdotally gave his expert opinion based on previous research.
Dr Humphries said most animals can cope with big changes in temperature, as long as the change is introduced slowly.
“If the change occurs over 24 hours they will be fine,” he said.
Last week Lake Cathie hit the required factors to be artificially opened to the ocean by Port Macquarie-Hastings Council.
The current water level is 0.05m AHD and salinity is between 40 and 42 ppt, therefore a lake opening trigger had been reached according to council director Melissa Watkins.
However the lake will not be opened by council due to several risks associated with the action.
Ms Watkins said the risks include low lake water levels which limit the success of an excavated channel remaining open, the likelihood of excessive sand build up in the recently dredged area and potential risk for red weed to enter and become stuck in the lake should it naturally close quickly.
Another risk is the likelihood of insufficient flushing of the lake and system shock when very cold ocean water meets warm lake water.
The community is reminded to report any observations of fish kills to Fishers Watch by calling 1800 043 536.