Five African lions broke out of their enclosure at Sydney's harborside Taronga Zoo on Wednesday, triggering an emergency lockdown as police and specialist handlers scrambled to round up the pride.

Alarms blared throughout the zoo, a tourist drawcard that was closed to the public at the time, before the lions were tracked down and returned to their exhibit.

The zoo's executive director Simon Duffy told reporters one lion cub had to be tranquilized.

"This is a significant incident and a full review is now under way to confirm exactly how the lions were able to exit their main exhibit," he said.

"We don't have the exact details of how and why that occurred."

A statement from the zoo later said the escaped lions were observed in a small area adjacent to the main exhibit, where a six-foot fence separated them from the rest of the zoo.

"All persons on site were moved to safe zones. Four of the lions calmly returned to their dens, and one cub was safely tranquilized. The cub is now awake and well. All animals are now in their back-of-house exhibit and are being closely monitored."

Taronga Zoo -- set in a leafy, affluent neighborhood with views of Sydney's famous opera house and harbour bridge -- said it would be opening as normal later in the day.

Zookeepers were seen probing the fences of the lion enclosure for signs of damage.

The zoo has two adult lions, mum Maya and dad Ato, as well as their five cubs named Khari, Luzuko, Malika, Zuri and Ayanna.

A fully-grown male African lion can weigh as much as 250 kilograms, while females get as heavy as 180 kilograms.

A two-year-old Bengal tiger escaped its enclosure at Taronga Zoo in 1946, roaming the grounds in what a newspaper described as a "frenzy of fear" before it was shot and killed by a zoo keeper.

Some 20 police were rushed to a suburb in Sydney's west in 1982 after a lion was spotted near a soft drink factory -- the culprit later identified as a pudgy cat called Ginger.

One of Sydney's most famous animal escapes occurred in February 2020, when baffled onlookers watched three baboons scampering about an inner-city car park after they bolted from a medical research facility.

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© Agence France-Presse