The octopus, one of the aquarium's star attractions, had first vanished mysteriously from his tank late last year, and was missing for five days until a baffled staff member spotted him making a dash for the door.
Sid had been using a seawater drain- used to pump fresh sea water through the aquarium- in a bid to reach the facility's sea water tanks outside.
Since Sid was returned safely to his tank, the sliding doors providing access to his aquarium tank have been kept tightly closed.
But once an aquarium octopus has learnt to escape, it tends to persist.
And Sid duly slipped away twice more last month, apparently spending a few hours in the drains before being returned.
Tessa Mills, manager at the New Zealand Marine Studies Centre, which incorporates the Westpac Aquarium, said it was unclear exactly how Sid had got away, although an octopus can squeeze through the tiniest of gaps.
At times, staff had resorted to tying the doors shut with string. "We don't think he's learned to untie knots yet," she said.
Aquarium senior aquarist Matthew Crane said an octopus's intelligence made escape-proofing its tank "quite a challenge".
The release yesterday had gone smoothly, he said.
Aquarist Dr Jean McKinnon said Sid was generally "laid back" in his personality, but there was a strong mating instinct, and he was nearing the end of his two-year life expectancy.
The aquarium's next challenge is catching another octopus to replace Sid.
