From the zoo's official site:
An initial review of this morning’s incident has confirmed that an integrity issue with a containment fence enabled five lions to temporarily exit their main exhibit.
CBS has a little more:
Taronga Zoo — set in a leafy, affluent neighborhood with views of Sydney's famous opera house and harbor 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....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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From the Guardian's write-up of that baboon escape:
These particular baboons are in a family group of seven – the three who escaped and four other females. The NSW health minister, Brad Hazzard, said the group would be reunited on Friday, after the male’s vasectomy, which was delayed by a day.Hazzard said the male and “two of his wives” escaped due to a faulty lock on the truck. It’s unclear how many locks were between the baboons and freedom.