Spring rains and hot days have heralded the start of Sydney's yearly mosquito invasion, with residents in riverside suburbs in particular feeling the bite.

Helicopters are spraying mangrove forest and saltmarsh areas along the Parramatta River at Homebush in a bid to drive down "nuisance" levels of the pesky insects as the weather heats up.

"Nuisance" levels of mosquitoes have prompted the Sydney Olympic Park Authority to start its annual spraying program to reduce the area's population. Credit:Reuters

Dr Cameron Webb, a medical entomologist from NSW Health Pathology, said: "Mosquitoes, like most insects, love warm weather.

"As soon as we start to get the first of those warmer days, mosquitoes spring into action."