The Dominion Post had an interesting Boxing Day feature (26/12), part of a series they are doing over the holidays on undiscovered animals. Reports over historic and modern times, including sightings by renowned people like Captain James Cook and Julian von Haast, attest to the presence in New Zealand of a native beaver or otter. NZ Maori described an animal (waitoreke? or kaureke?) to Dr David Monro (a Scottish doctor and later politician) living at the source of the Clutha and building ‘lodges.’ Much earlier crewman on Resolution saw and described an animal in Fiordland, about which Cook wrote,
“The most notable conjecture is, that it is of a new species. Be this as it may, we are now certain that this country is not so destitute of quadrupeds as was once thought.” ~ A Voyage Towards the South Pole and Round the World (1777).
Von Haast, later inaugural curator of the Canterbury Museum, reported otter-like tracks near Ashburton as well as a sighting by sheep farmers.
This is of interest, because NZ has no natural mammals other than a rare bat and marine dolphins and whales. The rest are all birds and an unusual reptile related to the dinosaurs, the tuatara (living dinosaur) that lives in a burrow.
It is also of interest, because of a fossil in Otago showing a native mouse, and the presence of mice and Polynesian rats, the kiore, is vital to the dating of people in New Zealand. Certain anomalies surrounding the presence of mice/rat bones in NZ shift the potential date of human colonisation to this last most extreme land mass in the story of human migration (currently ca. 800-1200 ad). The Otago fossil suggests a native mouse may have been indigenous, assimilated or made extinct by later arriving Polynesian kiore (or were they here all along and therefore the arrival of Polynesians to NZ was later?).
If a native mouse did exist, as proven by the fossil, then could native mustelidae (otters) or castoridae (beavers) have lived in NZ too? The world’s largest birds -several species of moa- made it here with the tuatara as New Zealand broke off like an island ark from the great paleolithic landmass of Gondwanaland.
I think their presence here is unlikely. If a fresh water or marine beaver or otter survived in New Zealand and floated eastward into the South Pacific on our large land mass with birds, we would expect other mustelidae -such as ferrets, stoats, weasels and certainly rats (the most adaptable of all creatures, and fast breeding)- to exist too. But none do. Moreover, there is so far no evidence of extinct creatures in NZ of this type (as with moas and eagles). And, how would the awkward and slow breeding kiwi survive in the same environment and eating the same food source as these predatory killing machines? It is not credible.
The complete absence of these creatures into historical times, or any evidence of them in the past, other than vague reports and sightings, like UFO mother ships, suggests they did not make it to the extremes edges of Gondwanaland then to break away as part of this unique island sanctuary. Prof. Arnold Wall was intrigued by the reports and gathered all the information he could, concluding that it did not exist.
- In 1848 Walter Mantell asked Maori for the skin of a kaureke said to live in Canterbury, but one was never provided.
- In 1855 Rev. Richard Taylor published accounts of beaver-like creatures in Fiordland, matching the reports of Cook’s crew in the same area.
- In 1861 The Press in Christchurch published a report at Bottle Lake.
- In 1880 a animal fitting the description of an otter was shot in Southland.
- In 1939 two amateur naturalists described seeing an otter-like creature in Te Anau.
- In 1971 a hunter makes a last report. Nothing has been seen since.
If they did exist, their young were perhaps predated, like many of our native birds, by introduced rats.
It remains feasible, but I suggest unlikely, and if the reports are real, are likely mistaken identity, perhaps kiwi, or weka in low scrub.

Hmm… the waitoreke myth is interesting (though, as you suggest, likely nothing) but a lot of details not quite right here.
Tuatara aren’t particularly closely related to dinosaurs, in fact birds (which are “living dinosaurs”) and crocodiles are much closer relatives to the extinct dinosaurs.
The St Bathans fossil (which I presume is what you are referring to with “otago fossil”) isn’t a mouse, in fact, it’s less closely related to mice than kangaroos or platypuses are. There is little reason to think it survived into modern times – much of the St Bathans fauna (crocodiles, eucalypts and flamingos..) don’t have descendants here. In any case, it has no bearing on the age of kiore bones, which has largely been settled now anyway. .
Finally, the idea that New Zealand is “a unique island scanctuary” with remnants of Gondwanan flora and fauna has taken a hit in recent times. Very few of our plants or animals can be reliably traced back to a Gondwanan origin, most have been shown to be recent arrivals.
I have long ago lost my copy of ‘Hidden Water’ by Philip Houghton. I seem to remember it carried an account of a sighting in the Hollyford / Big Bay/ Pike area. It intrigued me. At that time [1970's] I was meat hunting in S Westland. I questioned a few mates who used to work that area. One reckoned he had seen tracks and he was mostly stable and not given to bs.