An important traditional food source for Māori (who name them ōrea), longfin eel numbers are declining and they are classified as endangered, but over one hundred tonnes are still commercially fished each year.

[4][7] Like other members of the Anguillidae, longfin eels have a rather unusual life cycle: they grow and mature into fertile adults in fresh water, then migrate to the sea to breed.

[4] The mature eels then die, their eggs floating to the surface to hatch into very flat leaf-like larvae (called leptocephalus) that then drift along large oceanic currents back to New Zealand.

[4] These occupy estuaries for their first year, during which they develop colouration and become elvers, which resemble small adult longfin eels.

[4][11][15] The recruitment of glass eels into New Zealand's freshwater river networks is a very variable process, thought to be affected by the El Niño and La Niña Southern Oscillations.

[17] Their diet as small eels largely consists of insect larvae, such as dragonfly nymphs (Procordulia grayi).

[18] Anguilla dieffenbachii is endemic species to New Zealand, widely distributed through the country's lakes and rivers, including the Chatham Islands.

[6][4][19] Longfin eels are often found great distances inland (up to 361 km) along fresh waterways and in high country lakes which are connected to the sea.

As juveniles, they prefer shallow water (under 0.5 m deep) with coarse substratum and faster than average stream flow (such as that found in riffles).

[21][4] Ōrea are an important traditional food source for Māori, who have long had extensive knowledge of the timing of their upstream and downstream migrations.

[30][31] In June 2012 it was reported that some pet food companies use the nationally threatened eels in their products, sparking outrage by conservationists.

[32] Scientists and conservation groups have growing concern for the survival of the species, as they can be legally killed and have a slow reproduction rate, breeding only once at the end of their lifetime.

[28][33] The 2018 IUCN assessment noted a sharp decline in longfin numbers over the last century, accompanied by significant reduction of habitat.