They aren't quite flying, but feral pigs are crossing major roads and even swimming through rivers as they invade Sydney's suburbs.

Authorities say the diseased and destructive animals have gone as far as Pymble and Hurstville while wreaking havoc on the rural fringes.

Hundreds of feral pigs are found and killed in Sydney each year by the Greater Sydney Local Land Services, which considers them a major biosecurity threat because they carry diseases such as leptospirosis and Q fever.

Biosecurity manager Steve Parker said the pigs would stop at nothing to get a meal.