The largest known animal to ever exist on Earth - with hearts the size of small cars - uses New Zealand waters as an important migratory route.
Reaching up to 30m in length and weighing up to 200 tonnes, Antarctic blue whales are larger than the biggest dinosaurs ever were.
The species' critically-endangered population today - an estimated 3000 - is a fraction of the 200,000 thought to have existed before the whales were hunted to the brink of extinction last century.
Now, a new study has shown how the ocean giants follow a route running through the South Taranaki Bight - also frequented by a blue whale species recently shown to be genetically distinct from those in the Pacific and Southern oceans.
Both species faced a range of threats, from climate change and plastic pollution through to ship strike and noise created by oil and gas exploration, which disrupted their communication.
In 2017, for instance, scientists captured recordings from an oil survey ship letting off seismic blasts every eight seconds, which was enough to drown out the blue whale's call.
In the new study, published today in the journal Frontiers of Marine Science, scientists used hydrophones anchored at the bottom of the ocean around central New Zealand.
They picked up the whales' low-frequency calls, which could be heard across hundreds of kilometres, to monitor their locations.
The whales were in their greatest numbers over winter, when they were northbound to warmer waters to breed, but also in spring, as they were returning from Antarctica to feed.
The big creatures were also detected to a lesser extent offshore from Kaikōura and Wairarapa.
"This research shows that New Zealand waters provide an important habitat for these incredible creatures," said the study's lead author, Dr Victoria Warren, of the University of Auckland's Institute of Marine Science.
"These animals are critically endangered, and we need to do everything we can to protect them."
The acoustic data pointed to the possibility that Antarctic blue whales may breed in New Zealand waters, since their calls were heard during the breeding season of September and October, but the evidence wasn't conclusive.
Kiwis might on rare occasion spot an Antarctic blue whale, but the creatures were notoriously difficult to distinguish – visually, at least – from the somewhat smaller, but still huge, pygmy blue whales.
To get a clearer picture of the habits of both types of blue whale, scientists eaves-dropped via microphones deployed at depths ranging from 100m to 1500m.
While identifying the sub-species visually could be difficult, it was more straightforward when relying on sound.
The data revealed that pygmy blue whales seem to congregate in the South Taranaki Bight, especially between March and May.
Underwater microphones were deployed in 2016 at four locations around central New Zealand - the South Taranaki Bight, Cook Strait, and off the coasts of Kaikōura and Wairarapa – and in 2017 at three of those locations.
Over 106 days, a total of 20,751 blue whale calls were detected, with both of the sub-species turning up at all of the locations.
"This research really illustrated the value of long-term deployments of underwater microphones, for monitoring rare and hard-to-observe animals such as large migratory whales," study co-author and fellow University of Auckland researcher Associate Professor Rochelle Constantine said.
The study also involved scientists from Niwa, Texas A&M University in the US, and acoustic monitoring firm JASCO Applied Sciences in Australia.