KEY POINTS:
The birds on Tiritiri Matangi Island haven't seen a rat in 15 years.
Despite the thousands of visitors who arrive on hundreds of ferry trips and uncounted private boats each year - each one a potential rat-carrier - not one rodent has made it to shore since 1993.
The result is a thriving bird paradise where takahe, kokako, penguins and kiwi live much as they did thousands of years ago - with the exception that they are now visited by 32,000 humans a year.
It's a remarkable turnaround for what was once a rat-infested island just north of Auckland - transformed by volunteers from barren farmland to dense native bush over 10 years.
The Department of Conservation (DoC) scientific reserve was a working farm until the 1970s; and before that the Kawerau-A-Maki and Ngati Paoa iwi lived there for centuries.
Volunteers began planting trees in 1984, forming the 1800-strong Supporters of Tiritiri Matangi Inc five years later when they began to run short of money.
Nowadays the group that calls itself "the supporters" is entrusted with delicate work normally reserved for experts and scientists.
DoC spokeswoman Liz Maire said the supporters had been around just one year fewer than DoC - 20 years last year.
"It's not just doing bits and pieces anymore," said Ms Maire. "They're doing work that used to be done by experts, in other words scientists and DoC people."
One volunteer, Simon Fordham, a supporter of 16 years who works as a medical supplies importer, is helping organise a transfer of native species from another island. His wife Morag, a volunteer guide on the island, works with nests of rare kokako.
British conservationist David Bellamy, a regular visitor to the island, was so impressed he said it deserved World Heritage status in his foreword to a book on Tiritiri Matangi.
These days, the island has so many rare birds it exports "spare" hihi and robins to other sanctuaries - a feat unthinkable not long ago.
The island's dawn chorus is so raucous, DoC ranger Dave Jenkins has only just learned to sleep in after 5am. Because the birds are protected they do not fear warm-blooded predators, so Mr Jenkins said people could get close to rare takahe or kokako before they would move away.
Ms Maire said the mix of species was as near as possible to what a coastal forest would have housed before humans arrived - an important factor in making sure the birds and the island's lizard inhabitants do not kill off each other.
It is hoped adventurous species such as kakariki and bellbirds will colonise the Shakespear Open Sanctuary, 4km away on the mainland, when it is completed late next year.
The supporters call Tiri a "reverse aviary" because humans are asked to stick to the tracks, while the birds have the run of the island.
The rangers and supporters are all too aware that just one careless visitor could bring the sanctuary into crisis. A ferry sailing was stopped in 2007 because its operators rightly suspected a rat was on board, saving the island from possible infestation.
"Come and enjoy the island but please don't bring your stowaways with you - please check your boat first," said Mr Jenkins.
Tiritiri Matangi
* Island bird sanctuary 4km off the tip of the Whangaparaoa Peninsula, or an hour's ferry ride from Auckland.
* A former farm that was planted with 280,000 trees in 10 years from 1984.
* Kiore (rats) were killed off in 1993.
* Managed by two full-time DOC rangers and an 1800-strong supporters society.
* Free to visit, apart from ferry fares. The ferry is limited to 150 passengers a day.
* Home to one of the oldest lighthouses in New Zealand, which has been solar powered since 1990.
* Stirred controversy in 2007 when the then-DOC ranger refused to kill three native harrier hawks DOC was concerned might kill kokako carrying the rare "Taranaki gene". The cull was authorised but the hawks have never been caught.
* Visitor displays and a $650,000 visitor centre were paid for and built by volunteers.
