"After we went for a dive, we went back to it and got a tape measure out, and it measured 4.2 meters (13.7 feet) long," diver Daniel Aplin told the newspaper.
Aplin said the squid appeared to be in good shape with no noticeable cause of death.
"It was pretty clean, nothing major on it. There was a scratch on the top of its head, but smaller than a lighter, tiny, wouldn't think that's what killed it," he told the Herald.
Aplin called a friend at the New Zealand National Institute of Water and Atmospheric Research, who arranged to investigate the death.
Giant squid can grow as large as a bus, according to National Geographic, but the invertebrates are hard to study because of their deep-sea habitat.
The largest ever measured was 59 feet long and weighed almost a ton, according to the science magazine.