Hunter Water's apologised for a bad smell that's lingering around the western side of Lake Macquarie.
Residents in Blackalls Park have been reporting a foul sewage odour in recent days.
The utility says it's been caused by an "unforeseen issue" with an aeration tank at the Toronto Wastewater Treatment Works, made worse by the recent hot weather.
"We sincerely apologise to those customers affected. We are working to resolve the issue as soon as possible," Hunter Water said in a statement.