The octopus - known as "Inky" - purportedly found a weak spot at the top of his tank, crawled out and creeped into a seawater runoff pipe that led to Hawke's Bay.
A manager at New Zealand's National Aquarium said on Wednesday that employees noticed the wet trail leading across the floor to the pipe.
"Octopus are really intelligent animals, very inquisitive, and they also tend to explore whenever they get the chance," said Rob Yarrell, according to AFP news agency.
Inky, a common New Zealand octopus, was found by crayfishermen and donated to the aquarium. Roughly the size of a rugby ball, he was nonetheless able to fit into tight spaces.
Reports were unclear about when exactly Inky made his great escape.