The remains of two new flightless bird species have been found in Central Otago by a trans-Tasman team of scientists.
The fossil bones of the extinct rails were located near St Bathans in sediments dating back up to 19 million years - before the takahe and the weka roamed New Zealand's forests.
The two species are tiny compared with today's rails - one is barely larger than a sparrow - and they are thought to be the oldest flightless rails known globally.
The study was led by Adelaide's Flinders University and also involved the University of NSW, Canterbury Museum and Te Papa, and has been published in the Journal of Systematic Palaeontology.
Rails (Rallidae family) include pukeko and coots and are usually seen around wetlands.
Many rail species fly, but the world largest flightless rails evolved in New Zealand, notably the takahe and weka.
Lead author Ellen Mather, a Flinders PhD student, says flightlessness in birds is often associated with an increase in size.
The weka, which is in the same family as study's fossil birds, is about the same size as a chicken.
However, one of the new fossil rails, named Priscaweka parvales, meaning ancient weka with small wings, was one-twentieth of the weight of a weka.
Small flightless birds exist only in the absence of terrestrial mammal predators and, when humans discovered New Zealand, the main islands had many flightless birds.
Dr Paul Scofield, from Canterbury Museum, says the discovery of the St Bathans rails raises the question of where they came from.
"The new species are unlike any rail known elsewhere so their exact origin or closest relatives remain a mystery."