High temperatures have been blamed for the deaths of hundreds of fish in two south coast waterways.
The Department of Primary Industries is investigating the fish kills at Meringo, near Moruya, and Wallagoot Lake at Tathra.
It is believed low water levels were the cause of hundreds of dead fish at Meringo, while a combination of low oxygen levels, high water temperature and changes to the salinity caused by recent rainfall led to mass deaths of fish at Wallagoot Lake this week.
NSW Upper House MP David Shoebridge reported hundreds of dead bream, eels, mullet, fingerlings and prawns were found in the saline coastal lagoon near Moruya, which is intermittently closed to the ocean.
"The suspected cause is related to the very low water levels at Lake Meringo and the resulting poor water quality this causes," a DPI spokesperson said.
"At these low levels, coastal lakes are very susceptible to high water temperatures, algal growth and low dissolved oxygen.
"Poor water quality in our coastal lakes is likely to continue without significant rainfall."
Mr Shoebridge blamed heat and drought, but also questioned wider policies.
“The record summer temperatures and low rainfall are the most obvious explanations for this fish kill. However, we can’t rule out the impacts of local land use as potentially contributing as well," he said.
“Seeing these thousands of dead fish really brings home the environmental damage we are doing to the planet and the scale of the challenge before us to deal with climate change."
Further south in the Bega Valley, one resident reported seeing "thousands" of dead bream in Wallagoot Lake on Tuesday, following the death of a number of spined leatherjackets on Monday.
Neil Fisher said the colour of the lake had changed from a "rusty red algae" to a khaki green colour.
Water quality monitoring across the lake on Monday found "extremely low" levels of dissolved oxygen and high water temperatures, which likely contributed to the mass fish deaths, Bega Valley Shire Council said.
A council spokesperson said low levels of oxygen formed in the lower layers of the lake were "turned over" causing the oxygen depleted water to come to the surface and impact aquatic life.
The lake will be monitored over coming weeks, with follow up testing of dissolved oxygen levels.
The DPI said while it does not have regular monitoring programs for coastal lakes, staff would keep an eye on areas of concern during normal fisheries patrols.
While Bega council had received inquiries from residents requesting Wallagoot Lake be opened to the ocean, a council spokesperson said it was not possible.
"The extremely low water levels in the lake and the distance from the entrance to the main basin of the lake, means that attempting an opening is not practical," the spokesperson said.
"The low dissolved oxygen levels could potentially be exacerbated if water was to drain from the lake, and the entrance closed on a low tide.
"The main basin of the lake is over 1km from the entrance bar and the area between is characterised by small isolated shallow pools of water. Any attempt to open would require extensive excavation with little chance of success."