Strong winds are causing havoc at Sydney Airport, with at least 20 flights cancelled after two runways were closed.

The airport is down to a single runway after air traffic controllers closed two runways at 2pm, causing lengthy delays.

Wind gusts are expected to reach at least 50 km per hour this afternoon.

At least 20 Jetstar, Virgin and Qantas flights have already been cancelled.

The winds are also causing problems at Melbourne Airport.

Travellers are being urged to contact their airline before arriving at the airport.