Key Points:

The first confirmed tuatara nest in over 200 years on mainland New Zealand has been discovered at the Karori Sanctuary in Wellington.

Sanctuary staff uncovered the four ping-pong-ball-sized leathery white eggs yesterday during routine maintenance work near the sanctuary's mammal-proof fence.

Raewyn Empson said that about this time last year staff had found a gravid (egg-carrying) female.

The eggs would have been laid almost exactly a year ago in a shallow trench dug by the female and then backfilled.

"We knew of two suspected nests but didn't want to disturb them to confirm whether or not they contained eggs."

The nest had been uncovered by accident and was the first concrete proof that the sanctuary's tuatara were breeding.

Ms Empson suggested there might be other nests in the sanctuary.

The eggs had been immediately covered up again to avoid disturbing their incubation.

Although only four eggs were unearthed, it was likely that there were more in the nest as an average clutch contained around 10 eggs.

Ms Empson said tuatara maternal responsibilities were to guard the nest for a few days after laying to prevent other females digging it up.

All being well, the tuatara could hatch any time between now and March.

The hatchlings would break out of the eggs using a special egg-tooth that falls off after about two weeks.

For the first six months or so, the legendary third eye, for which the tuatara was most famous, would be visible as a white patch on the forehead, she said.

The light-sensitive pineal gland on the top of the skulls slowly closes over as they mature.

As with some other reptiles, soil temperature would determine the animals' gender, said Ms Empson. Warm soil (over 21C) resulted in males, and cool soil (under 21C) females.

Tuatara have been found only in New Zealand and other species in its order became extinct about 60 million years ago, leading scientists to refer to them as living fossils.

It is not certain how long tuatara had been absent from mainland New Zealand but they were rare if not extinct by the late 1700s, mainly because of rats eating their eggs.

The re-establishment of a population at the rat-free sanctuary in 2005 was a breakthrough in re-establishing this species in the wild on mainland New Zealand.

Seventy tuatara were transferred to the sanctuary in 2005 from Takapourewa (Stephens Island) in the Marlborough Sounds. A further 130 were released in 2007.

Life cycle:
* Tuatara breed only every two to four years.
* Female tuatara become fertile at about 13 years of age.
* A female will carry up to 12 eggs for nine months.
* The leathery eggs are laid between October and December, buried and abandoned.
* After about 12-15 months the eggs hatch.
* From day one, tuatara take care of themselves.