A juvenile shark has been rescued and returned to the sea after it was found in ocean baths in Sydney's northern beaches.

The dusky whaler was discovered in Palm Beach rockpool on Tuesday morning, after it was possibly brought in with the tide overnight.

A handful of locals had already finished their morning swim by the time the shark - estimated to be about 80 centimetres long - was discovered.

The pool was closed while a team of marine biologists were called to retrieve it and carry it a few metres to the open water.

Hope Nugent from Manly Sea Life Sanctuary told the Manly Daily it took about a minute to grab the shark and release it.

"We don't know how it ended up in the pool, but there have been high tides and sometimes they come in on the high tide," Ms Nugent said.

"You do tend to get other animals, such as stingrays, but it's quite infrequent for sharks to get caught in there."

The shark posed no risk to humans, Ms Nugent said. When it was released, it quickly swam away.

Before the shark was removed, it provided an added tourist attraction for swimmers at Palm Beach.

A sign written in chalk was placed next to the pool, reading: "Warning, pool closed, shark in pool."

At least one tourist posed with the sign, quipping that it was very Australian.

Fairfax Media