North Sydney swimmers joined by a shark for their morning laps in ocean pool

Swimmers doing their morning laps at an ocean swimming pool yesterday had no idea what lurked beneath the surface.

North Sydney swimmer Jennifer Hill was swimming in the pool for more than an hour before she realised a shark had joined her for her morning swim, the Manly Daily reports.

"My initial response was obviously it wasn't very hungry because it had ample opportunity," she joked.

But she wasn't bothered by her toothy companion: "It was a beautiful experience.

"Everybody who swam in the pool this morning didn't know ... except one person."

Beachgoers were somewhat shocked to see the dusky whaler swimming in the popular Palm Beach pool with other swimmers, the newspaper reported.

Hill, of Bilgola, estimates she was swimming for more than an hour before a bystander alerted her a shark nearly 1m long was in the water with her.

Manly Sea Life Sanctuary's Hope Nugent, who helped return the creature to the ocean, said the dusky whaler was no threat to swimmers.

"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," she told the Manly Daily. "You do tend to get other animals, such as stingrays [ending up in ocean pools], but it's quite infrequent for sharks to get caught in there."

Nugent said it took about a minute to catch and release the shark, and it quickly "swam off".

"They do get stressed and there is lactic acid in their bodies they need to release."

Avalon's Rita Kluge photographed the shark as it cruised the pool.

"I couldn't wait to jump in but I wasn't allowed," she said.