Tractor driver Michael Johnston discovered the footprints in March.

A New Zealand tractor driver has stumbled along an "astonishing" paleontological find while taking his dogs for a swim at a river on the country's South Island.

Key points:

• Mr Johnston contacted experts after finding the moa footprints in the Kyeburn River
• The footprints were extracted from clay to be studied after water was diverted from the swimming hole
• The footprints could be up to 12 million years old

After noticing some strange markings in the clay in the Kyeburn River in March, local resident Michael Johnston alerted the Otago Museum of his discovery via a Facebook message.

Days later, experts from the museum dove into the river, finding seven moa footprints in the hard clay of the river, under a metre of water.

The Otago Museum described the imprints as an "astonishing preservation" of tracks made by a moa.

They are the first reported footprints of the extinct, flightless bird on the South Island.

An artist's impression of a pair of giant New Zealand moa being attacked by a raptor.

Mr Johnston's incredible discovery was aided by recent wet weather in the area.

"The last major flood event on the Kyeburn was in November 2018 and it's likely the event eroded the bank and exposed the clay slab containing the prints," said Otago Museum assistant curator of Natural Science, Kane Fleury.

Moa footprints have been found in at least 10 areas on the North Island, with at least 25 footprints recorded or extracted.

Each footprint is approximately 30 centimetres by 30 centimetres wide.

Otago Museum director Ian Griffin said the footprints could be up to 12 million years old.

On Friday, the footprints were extracted one by one from the soft, silty clay by experts from the Otago Museum and University of Otago, after water from the river was diverted from the swimming hole.

Mr Griffin said the site of the discovery was 3D scanned and mapped prior to extraction, with each footprint carefully extracted in order to prevent any damage to the specimens.

The moa footprints are estimated to be about 30cm long and 30cm wide.

"The whole museum team is excited about the find, and we are really, really excited to start the process of conserving the footprints for future analysis and study," he said.

The footprints will help aid future research into the species, which went extinct more than 550 years ago.

"These prints form an important path back to the past. We might imagine the moa pacing, perhaps slowly and deliberately, across a nearly-dried firm muddy surface. The prints may add to knowledge of moa size, body mass, stride length and speed," said Professor Ewan Fordyce, from the University of Otago's Department of Geology.