A hatchling of a rare reptile with lineage dating back to the dinosaur age has been found in the wild on the New Zealand mainland for the first time in about 200 years, a wildlife official said Thursday.

Karori Wildlife Sanctuary staff discovered the baby tuatara during routine maintenance work at the Wellington-based animal sanctuary, conservation manager Raewyn Empson said.

"We are all absolutely thrilled with this discovery," Empson said. "It means we have successfully re-established a breeding population back on the mainland, which is a massive breakthrough for New Zealand conservation."

Tuatara, which measure up to 80 centimetres when full grown, are the last descendants of a lizard-like reptile species that walked the Earth with the dinosaurs 225 million years ago, zoologists say.

There are estimated to be about 50,000 of them living in the wild on 32 small offshore islands cleared of predators, but this is the first time a hatchling has been seen on the mainland in about 200 years.

The New Zealand natives were nearly extinct on the country's three main islands by the late 1700s because of the introduction of predators such as rats.

Empson said the hatchling is thought to be about one month old and likely came from an egg laid about 16 months ago. Two nests of eggs — the size of table tennis balls — were unearthed in the sanctuary last year, and tuatara were expected to hatch around this time.

"He is unlikely to be the only baby to have hatched this season, but seeing him was an incredible fluke," she said.

The youngster faces a tough journey to maturity despite being in the 250-hectare sanctuary and protected by a predator-proof fence. It will have to run from the cannibalistic adult tuatara and would make a tasty snack for birds of prey, Empson said.

"Like all the wildlife living here, he'll just have to take his chances," Empson said.

"They've been extinct on the mainland for a long time," said Lindsay Hazley, tuatara curator at the Southland Museum and Art Gallery on South Island. He added that "you can breed tuatara by eliminating risk, but to have results like this among many natural predators (like native birds) is a positive sign."

About 200 tuatara have been released since 2005 into the Karori Sanctuary, which was established to breed native birds, insects and other creatures. Tuatara have unique characteristics, such as two rows of top teeth closing over one row at the bottom and a parietal eye — a dot on the top of the skull that is believed to be light-sensitive and is sometimes referred to as the animal's third eye.