Wellington  - The population of New Zealand's kakapo - the world's rarest parrot - has topped 100 for the first time since the birds were rediscovered 33 years ago, it was announced on Wednesday.

Kakapos were thought to be extinct until 1976 when a small population was discovered on Stewart Island and a breeding programme started to save the species at a sanctuary on nearby Codfish Island.

A bumper season has seen six chicks hatch while 24 more eggs are being closely monitored by a team of about 30 Conservation Department workers and volunteers, said Deidre Vercoe, manager of a recovery programme for the flightless, nocturnal birds.

The new arrivals more than double the kakapo population from the number known to be alive a little over a decade ago.

Although Codfish Island is predator-free, the new chicks remain vulnerable and will be hand-reared in a specialist unit to ensure their survival until they can be returned to the wild.

Vercoe told Radio New Zealand that passing the 100 mark was a significant milestone, raising hopes that the kakapo population was reaching a sustainable level.

The recovery programme has been a tortuous process as kakapos are not prolific, breeding and laying eggs only every two to four years.

Males, who deliver a sub-sonic mating boom that can be heard for kilometres, do not start mating until they are about five years old and the youngest females known to lay eggs are six.

Only 20 chicks survived in the first 30 years after they were rediscovered and those hatched last year were the first since 2005.

Vercoe said a bumper crop of fruit from native rimu trees, which kakapo mothers depend on to feed their young, spurred breeding this season.

Kakapos are not only the world's rarest parrots, they are the heaviest, with full-grown males weighing up to 2.2 kilograms and females 1.4 kilograms.

Their size and inability to fly is believed to have contributed to their near-extinction at the hand of predators, the camouflage of their mottled moss-green feathers being their main form of defence.

The parrots sleep all day, usually in hollows in the ground, and spend the nights on solitary meanders through the bush feeding on seeds, roots and berries.

They are the only representative of a unique sub-family of parrots with the scientific name Strigops habroptilus, which experts say has a combination of biological features not shared by any other species.

Survival of the species has depended on Department of Conservation staff and volunteers who have identified all the birds and keep nightly vigils over their nests when eggs have been laid.

They camp near the nests to watch that the females incubate properly, often covering the eggs with heat pads when the mothers go foraging for food. (dpa)