According to a Horizons Regional Council article: DNA techniques reveal the full spectrum of river life:
one of New Zealand’s rare native fish Lamprey (Geotria australis) has been discovered in waterways like the Waikawa Stream and reaches of the Ōhura River in the Horizons Region.
The article also says:
A technique called environmental DNA or eDNA for short, is helping … identify the animal species present in our waterways. …
DNA is shed by aquatic life from their skin, scales, fluids and faeces. The technique is able to detect even minute traces of each species and the overall proportion of it within the water sample. The species of fish, birds, mammals, reptiles, plants, fungi, bacteria, insects, snails, worms and anything else are identified by comparing the DNA fragments to a reference database. This requires as little as a mug of water sucked through a filter, though more water increases the chance of detecting organisms.