Five lions were spotted outside of their main enclosure 6.30am last Wednesday, which triggered a major emergency response.

A loud duress alarm - sounded soon after the discovery of the lions outside their normal enclosure - prompted a strict lockdown, which resulted in one lion having to be tranquilised and four others being guided back to safety.

Today, zoo staff confirmed that although an incident review is ongoing, the structural fault in the fence was to blame.

"Preliminary independent engineering advice has confirmed that swages (clamps that join wire cables together) failed, enabling a lacing cable that connects the fence mesh to a tension cable to unravel," Taronga Zoo said in a statement.

"The lions were then able to create and squeeze through a gap.

"The preliminary review has recommended the engagement of an independent tensile-structure engineer to conduct detailed investigations and provide specialist advice on the failure and repair."

The review also found that the lions "played and interacted" with the fence for about 20 minutes before four cubs and later an adult male, Ato, were able to break through it.

One lioness, Maya, and one cub chose to stay inside the exhibit.

"Lioness Maya and the cub who chose to remain in the exhibit were recalled into their dens by keepers," the zoo said.

"The emergency response including staff in multiple vehicles was established to monitor and control the situation.

"This enabled the keepers to use their relationships with the lions, as well as their training for such incidents, to calmly call the lions back to their exhibit, ensuring a peaceful and safe outcome for all people and animals."

Taronga said it will continue to review the incident and share more information when it becomes available.

"The lions will continue to remain in an outdoor, back-of-house holding area pending the specialist engineering advice," the zoo said.