Humpback whale spotted in Sydney Harbour

A whale has been spotted in Sydney Harbour, halting ferries and putting on a show for tourists.

The humpback whale dazzled tourists and locals in Circular Quay this morning, with ferries halting operations momentarily.

Meanwhile, NSW maritime crews sailed alongside the whale to escort it back out to sea.

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Dr Vanessa Pirotta, a marine scientist working with NSW Maritime, confirmed the sighting on her social media accounts.

She said a whale in the harbour is beautiful to see, it poses a "navigational nightmare" for passing boats.

New South Wales Water Police, the Port Authority and Transport for NSW urged all vessels on the harbour to keep an eye out for the whale and give it space as it was escorted out to sea.

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The humpback is one of 40,000 migrating to Queensland to mate and give birth.

Their annual journey spans 10,000km, starting from Australia's east coast in April and ending near sunny Queensland by November, according to the Department of the Environment and Water Resources.

It's not unusual to spot a whale off the NSW coast in these migration months, with two spotted leaping from the sea in Lennox Head earlier this month.