Passengers travelling to Sydney and Melbourne have been warned to expect delays as a number of flights have been cancelled due to gale force winds.

Approximately 20 flights have either been cancelled or delayed, as a result of Sydney Domestic Airport only having one single runway open, a Sydney airport spokesperson confirmed.

A total of 12 Virgin and 12 Qantas flights have been cancelled, with several others hit with massive delays.

Damaging winds, averaging 60 to 70 km/h with peak gusts around 90 km/h, are causing havoc in Sydney, and parts of NSW.

Customers are urged to check their flight status for updates throughout the day.