Using a GoPro camera strapped to his helmet, the man films himself leaping off Jump Rock at Collins Beach in Manly.
No sooner has he got in the water, his friends can be heard shouting "Shark!".
The film then shows the shark swimming dangerously close, and the man's quick scramble to get out of the water.
Shark expert Rob Townsend from Manly Sealife Sanctuary said he wouldn't be surprised to see a great white in the harbour - especially at Collins Beach.
He told the Daily Telegraph: "Collins Beach is reasonably close to the heads, so a great white could find its way into the harbour.
"They are solitary creatures and swim up and down this coast. You can come across them pretty much anywhere, though I can't recall hearing of one in the harbour before."
But many people have declared the footage a fake. People posted comments on YouTube, with one writing: "Gonna go out on a limb and say fake, see 0:59 and you will see why."
Another says: "The shark Teleports at the 0:59 mark..... It goes from centre of the screen to the left of the screen in one frame..."
Do you think this is a real encounter? Leave your thoughts below.
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