Sydney’s famous Circular Quay was noticeably missing some key features this morning as the iconic Harbour Bridge and Opera House sails vanished into thick fog.

Multiple ferry services were delayed and cancelled, and more than 20 regional and domestic flights out of Sydney Airport were affected.

At around 2am on Thursday, visibility at the aiport was down to 400 metres, the Sydney Morning Herald reports.

The poor visibility caused delays and cancellations for early local flights, and airlines including Jetstar, Tigerair, Virgin and Rex were affected, but no international flights were interfered with, a Sydney Airport spokesperson told Broadsheet.

There is likely to be a flow on effect to domestic services throughout the day and passengers are advised to check with their airlines for flight-status updates.

Other parts of Sydney were also engulfed by the thick fog, believed to be caused by clear skies, old winds and the last few days’ heavy rain, Weatherzone meteorologist Graeme Brittain told the SMH.

Western Sydney was the worst affected. Visibility in Richmond, Campbelltown and Bankstown dropped to 50 metres, and in Homebush it dropped to 100 metres.

Early-morning Paramatta River, and Circular Quay to Manly ferry services were replaced by buses, but all lines have now resumed normal services, a NSW Transport Management Centre spokesperson confirmed to Broadsheet.