By mid-afternoon, the fog had enveloped Sydney's coastline from the Northern Beaches to the Royal National Park on the city's southern outskirts.

Weatherzone senior meteorologist Jacob Cronje said a number of factors combined to create the perfect conditions for the unusual event.

"You have the warm moist air mass over warmer than normal seas and weak winds; it's the perfect recipe for this kind of thing, but it's not something that's expected at this time of the day," he said.

"We’ve had a lot of rain and a bit of a cooling air mass over the ocean, and when it comes in contact with that warm sea surface air above it, condensation occurs.

"Then with those onshore winds, weak as they are, they're not mixing the fog. They're just strong enough to push it onto the coast."

Mr Cronje said that, while it was hot and sunny – it was 27 degrees at Sydney Airport at 1pm – the "soup"-like air and weak south-westerly wind was keeping the fog in place.

While he expects the fog to lift when the wind strengthens later in the afternoon, he said some parts of the coast may see fog well into the evening until the warm air mass cools down.

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"I expect the winds to strengthen later today, so I imagine by this afternoon from about 5pm we should see some breakup, however having said that it could last at some parts well into the evening.

"We are expecting a weak southerly change tonight or early tomorrow morning."

He said it was "fascinating" to see fog hugging the coast in the middle of the day.

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"It’s not common at all. It doesn't mean it doesn't happen – we occasionally see harbour fog – but generally not this time of the day, this is quite uncommon," he said.

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