Sydney residents woken by ship's horn as fog blankets the city

Weary Sydneysiders have been searching for answers as to why they were woken up in the early hours of Thursday morning by a loud horn.

Frustrated eastern suburbs residents took to social media when the siren, coming from Sydney Harbour, first went off at 5am.

“Anyone know why there’s a ferry/ship sounding its loud a** horn in habour constantly since 5am?” one local resident asked.

“Sounds like it’s in my bedroom as I’m just down the road,” another wrote.

Others began sharing images of the heavy layer of fog blanketing the harbour.

A spokesperson from the Port Authority of New South Wales told Yahoo7 News that the horn was not coming from a stranded ship but rather the Carnival Spirit.

“There is an international maritime regulation where they have to sound the horn every two minutes,” the spokesperson explained.

She said the thick fog, that later saw all Sydney ferry services cancelled, had caused extremely low visibility for the cruise ship.

Unable to make its way into the overseas terminal, the cruise ship was forced to sound its siren repeatedly as a warning to other boats.

The cruise ship was later cleared to proceed to the terminal after the fog cleared.

Planes continue to depart and arrive at Sydney Airport, but Airservices Australia advises passengers to check with their airlines for any cancellations.

Commuter relief as ferries recommence

Some Sydney ferry services including the Manly ferry have resumed after thick fog cancelled all routes.

Low visibility in Sydney Harbour and the Parramatta River halted all services until just before 7am when a cross harbour service briefly resumed.

It was suspended again minutes later before services along Parramatta River and from Neutral Bay resumed.

The Manly to Circular Quay service resumed just before 8am.

Planes continue to depart and arrive at Sydney Airport, but Airservices Australia advises passengers to check with their airlines for any cancellations.