Why one area of Sydney has been swarming with mosquitoes

For weeks, residents in south-west Sydney have complained about a plague of mosquitoes. Relief could be at hand within the next fortnight, as authorities deal with the main infestation in a wastewater facility in Liverpool.

Mosquito expert Dr Cameron Webb said the mosquito numbers in south-west Sydney were two to three times higher than usual for the time of year, and the culprit species was the “brown house mosquito”.

Residents in Chipping Norton and nearby areas have borne the brunt of the nuisance, but the mosquitoes have also been spotted swarming inside train carriages travelling from south-western Sydney to the city.

Sydney Water has been working to kill larvae and remove frogbit weed from the oxidation pools at the Liverpool Water Resource Recovery Facility, which provided a perfect breeding ground for the pest.

“Operational staff and licensed contractors have deployed boats, helicopters, excavators, sucker trucks and other specialised machinery to clear the aquatic material contributing to mosquito breeding,” a Sydney Water spokesperson said.

“To date about half the vegetation has been removed. Field assessments show a 98 per cent reduction in larvae across 10 of 12 sites since Friday.”

Before-and-after photos of the weed being removed from Liverpool Water Resource Recovery Facility.Credit: Dr Cameron Webb

The spokesperson said the impact of mosquitoes should be reduced by 90 per cent within a two-kilometre radius over the next two weeks.

Webb, who works with NSW Health Pathology and the University of Sydney and has been involved in the response to the outbreak, said he was pleased with how Sydney Water had responded. He expected the control measures to work, but would monitor the situation to make sure there was not a resurgence over summer.

Webb said most of Sydney was experiencing a normal number of mosquitoes. However, the insects were more noticeable in October and November because people spent more time outdoors, there was generally water in the environment for the larvae, and warmer weather increased mosquito activity.

September and October had typical temperatures and rainfall across Greater Sydney, weather bureau data suggests, though it is tipped to be a warm, wet summer. Humidity increases with global warming, and international data suggests 2024 is shaping up to be the hottest year on record.

Brown house mosquitoes were only considered a minor threat to public health in Australia, Webb said, and typically bred in stormwater and wastewater. Other species bred in saltwater or stagnant water in bushland.

The main mosquito-borne disease that affects Sydney, mostly at the fringes, is Ross River virus. Webb said there were about 5000 cases across Australia.

Liverpool Council has been spraying public facilities such as toilet blocks and septic tanks. Backyards including swimming pools and pots can be secondary sources of mosquitoes.

The Morning Edition newsletter is our guide to the day’s most important and interesting stories, analysis and insights. Sign up here.