A bellbird sits 2m away. His body is still and taut, his head raised and he's putting mammoth effort and energy into singing. The song is similar to that of a tui, only more beautiful; the bell-like notes are clear and pure. He has great pitch and reach for such a tiny bird.

Three of us are following the old fisherman's track through a deep bush-clad valley in the middle of Tawharanui Regional Park. We stop to listen to this little green-grey fellow then, when we walk off, he flies ahead of us, finds another trackside perch, and starts his serenade again.

Tawharanui Regional Park is on a finger of land stretching into the Hauraki Gulf pointing towards Great and Little Barrier islands. There is a necklace of pretty beaches near the road end, on the sunny north side, and steep forested cliffs on the south side facing Kawau Island. The old fisherman's track, latterly named the ecology walk, crosses this peninsular, meandering up a valley through stunning rainforest.

Massive puriri reach the sky. Their broken, but still growing, lower branches lean across the stream and provide home for hundreds of mossy epiphytes and shy orchids. There are glades of nikau with 100-year-old giants standing tall. Thousands of their offspring make elegant patterns with curved fronds; little ones stroke our legs as we walk. Ponga unfurl lacy leaves, vines hang from high and, among all this verdant luxury, bellbirds chime, fantails flit across the path, black robins hop about, kereru feast on flowers and tui chase each other, weaving through the trees. And there are kiwis too; they love this moist, shady valley but don't present themselves to day-trippers.

In 2003 a predator-proof fence was built across the bottom of the peninsular and Auckland Regional Council started a concentrated campaign to rid the park of introduced animals that had been decimating the native birds and fundamentally changing the ecosystem for 200 years.

By 2005, Tawharanui was declared rat free ? the cats, stoats, weasels were eliminated a few years earlier ? and, as a reward for all the hard work, a flock of 100 or so bellbirds flew over from Little Barrier Island and made Tawharanui their home. Matt Maitland, open sanctuary co-ordinator, tells me it's the first time there have been bellbirds on the Auckland mainland in more than 100 years. These birds have been happily breeding and now there are about 800. In May, 100 were taken to Waiheke, Motuihe Island and Hamilton Gardens to start populations there.

The kiwis had human help to re-establish. The BNZ Operation Nest Egg programme, which funds captive hatching and rearing of kiwi chicks, introduced 44 kiwis in 2006. In the past two seasons, 11 chicks have hatched and survived naturally, probably more Maitland thinks, but they are shy and hard to count.

The fisherman's track scurries down a cliff to a stony beach so we turn east through regenerating bush and walk towards the tip of the peninsular. We don't see any kiwi but it's nice to know they are there. It's nice to know, too, that green geckos and forest geckos, whose populations had been ratted to the point that they were undetectable, are making a comeback and are often seen in this dense manuka-dominated scrub.

Never has manuka looked more beautiful. It's covered in white, delicately fragrant blossoms and, because of the absence of possums, patches of native clematis joyfully clamber over the manuka adding big, bright bouquets of white star flowers.

At Takatu Pt the wind is equinox-blustery, waves swirl against rocks and clouds change shape on top of Little Barrier Island. Flax and wind-bent manuka thrive and pohutukawa audaciously cling to the rocky windswept ledges. We picnic while watching seabirds, gulls and grey-faced petrels soar on the wind. The seabirds that nest on these cliffs also enjoy life without rats.

Most of the walk back to the carpark is across farmland. Paradise shelducks honk as we pass through their territory. It's spring and they're mating, nesting possibly, so they're more than usually ornery.

Skylarks, always a joy to hear, exuberantly twitter as they flutter high in the sky.

We walk down to the first of a string of crescent beaches at the land end of the park and watch dotty little dotterels trotting around on the sand and pecking at insects living in rotting seaweed. Dotterels are an endangered species but Tawharanui is adding to their numbers. Since de-ratting, the 10 resident pairs fledge between 10 and 17 chicks each year. The chicks don't stay but fly away, find partners, then nest on other north-of-Auckland beaches. Total dotterel numbers are about 1500 and are slowly increasing.

As we leave we stop at the lagoon near the park entrance to see what birds make their home around this deep brackish water. The lagoon hasn't always been a lagoon. Until the 1960s it was a quarry. It's now home of pateke (brown teal), paradise shelducks and mallards who all swim away in one multiracial ducky flotilla as we approach. There are also wading birds: oystercatchers and pied stilts, long-legged and delicate, pecking in the shallows.

We admire a humble shag, standing fearless on a tree stump. Shags aren't star birds like kiwis or endangered, but they have to be respected for their patience; they will wait perfectly still, sitting on a rock, wings spread and drying and are smart beady-eyed fish hunters.

Ten years ago I associated Tawharanui with coastal walking and great beaches. Now, for me, its main attraction is birds.

A gentle four-hour walk includes thick bush, coastal scrub, sea cliffs, beaches, pastureland and the lagoon: each of these varied ecosystems provides a perfect environment for different bird species.

Tawharanui has only been free of predators for five years, and already the increase in bird life is astonishing - and it's getting better.

I leave with an immense sense of optimism; our little corner of the world is becoming increasingly lovely.

If you go:

Tawharanui Regional Park is on the east coast, 1 hours' drive north of Auckland. More information at arc.govt.nz