It's not much to look at: one storey high, with an open doorway and a wood-framed window to either side; a pane on the left is cracked. There's a corrugated iron roof, rust-red in places – a recent addition, to keep the interior dry. And inside? No light, a dusty floor, some tools in the corner.
It's not much to look at, but what it looks at is beautiful. The tiny building faces its own small bay, draped with a profusion of greenery along curving shores. The trunk of a tree, scoured white by the salt water, lies half-buried in the sand, and there's a plank or two of what was once a jetty visible nearby. The sand itself is pinky-orange and strung with seaweed; the only sound is a twitter of birdsong and the lapping of water.
The oldest building on Stewart Island was constructed from stone and clay in 1835 by a retired whaler named Lewis Acker. The place where he built it – Harrold Bay – lies along a scenic walk east of the township of Oban, the only real settlement on Stewart Island and home to most of its 400 permanent residents.
Hidden down a private road close to Port of Call, the pretty B&B where I was staying, Harrold Bay is still a place of solitary wonder. As I walked to the shoreline, it was easy to make the mental leap back in time, to when Acker himself was earning a living as a boat-builder and coastal trader here. He married a Maori woman called Mary Pui; nine children were raised in this simple house before they moved away in the 1850s. A large family would, one imagines, have been some consolation for the loneliness of life out here 160 years ago.
Beyond the bay, the 30km-wide Foveaux Strait separates Stewart Island from New Zealand's South Island. It's an often treacherous stretch of water – a miniature version of the mighty Bass Strait, which does the parallel job of dividing the island state of Tasmania from mainland Australia. As with Tasmania, Stewart Island's landmass is diminutive in comparison with its near neighbour. Just 64km long and 40km across at its widest point, and barely bigger than Greater London, it may be the third-largest island in New Zealand's Pacific archipelago, but it's a very distant third.
The Maori people called this place Rakiura, which roughly translates as "Glowing Skies": occasionally the Southern Hemisphere's version of the Northern Lights, the Aurora Australis, can be seen overhead. The next landfall for travellers heading southwards is Antarctica.
The island also plays its part in the Maori creation myth of Maui, who was said to have raised a great fish (North Island) from his canoe (South Island). Stewart Island was supposedly the anchor for Maui's boat – appropriate for a place that still seems firmly rooted to the ancient past.
Shaped like a foot with a broken heel, Stewart Island has 700km of coastline, but only 20km of roads. Most of these are concentrated around Oban, a tidy little place on Halfmoon Bay, which seems to have one of everything the traveller might need, including a tourist office, a museum, a hotel, a pub and a fire station. It's here that visitors arrive after the one-hour ferry crossing from Bluff on South Island; a landing strip for light aircraft scratched on to a hilltop nearby provides an alternative transport solution for those – like me – who prefer not to brave the waves.
Indeed, it's from the air that Stewart Island reveals itself. All but one-sixth of the land is protected by national park statues, with lush hills and valleys forming a point at Mount Anglem in the north. Even though it doesn't match the grandeur of South Island's highest peaks, at 979m it's one metre higher than England's tallest mountain, Scafell Pike.
So densely packed is the crush of vegetation that for the most part it feels as if man has scarcely intruded here. It's like visiting Conan Doyle's Lost World, except that instead of being attacked by pterodactyls, hikers who choose to follow the three-day 29km Rakiura Track along the coast are likely to see birdlife that is either rare or extinct on mainland New Zealand: a kiwi, perhaps, or yellow-eyed penguins, or the predatory, flightless weka.
But really, twitchers need not trudge too far from Oban to be wowed by the avian spectacle. The tiny button of Ulva Island lies within Patterson inlet, a deep gash of water that slices through to the interior just to the east of the township. Here, predators such as cats and possums have been eradicated, creating a natural sanctuary. As a consequence, Ulva's neat walking tracks resound to the songs of birds.
Ian Wilson, my host at Port of Call, took me on the five-minute trip over to Ulva in his water taxi. Aside from a pair of Department of Conservation workers, I had the place to myself: kakas (forest parrots) squawked overhead as I strolled past wood-lined bays; rare South Island saddlebacks zipped along at eye-level, and I even had a close encounter with a perky Stewart Island robin.
But it was the song of the tui that impressed me most. Back on Stewart Island, as I carried on past Acker's house, I came across several of them, black-frocked, with a tuft of white at their throats revealing why they were once called Parson Birds. But no vicar ever sermonised like this: they emit an entirely unecclesiastical series of beeps, squeaks and whistles – often imitations of the calls of other birds – that sound like R2-D2 from Star Wars after a particularly rough night.
A few minutes later, the cliff path ended at Acker's Point. Here a small lighthouse indicates the eastern extremity of Halfmoon Bay. Beyond the crashing surf I could see a few splashes of green dotting the sea. The Maori people still camp on these outlying islands for several weeks every April and May to harvest the chicks of the sooty shearwater (or muttonbird) for food.
Aside from harvesting the interest of tourists, many locals back in Oban spend their time fishing for crayfish and blue cod. It's a lively community, as I discovered later that evening in the crowded bar of the South Sea Hotel, where Ian and his wife Philippa plied me with fish and chips. Ian was particularly proud of being a sixth-generation islander, and hopeful about future plans to eradicate non-native mammals from Acker's Point, thereby increasing the likelihood that the native birdlife would thrive in his garden.
The next day he took me to Lee Bay. The asphalt ends here, and over 200km of walking tracks – including the epic 10- to 12-day North-west Circuit – begin. Beside a broad scoop of beach stood a rather literal sculpture constructed to mark the inauguration of the national park in 2002. Huge links in a chain disappeared into the sea, to signify Stewart Island's role as an anchor for the rest of New Zealand. Ian pointed out the bullet hole in the top of the sculpture; apparently a local took a dislike to the concept and had a pot-shot during the unveiling ceremony. Perhaps the protester had a point. After all, being cast adrift rather than chained to the mainland is part of what makes Stewart Island so appealing.
Travel essentials: Stewart Island
* The only direct flights from the UK to Auckland are on Air New Zealand (0800 028 4149; airnewzealand.co.uk ) from Heathrow via Los Angeles or Hong Kong. The airline has connections to Invercargill on South Island.
* Stewart Island Flights (00 64 3 218 9129; stewartislandflights.com ) runs a service from Invercargill to Oban three times daily.
* Stewart Island Experience (00 64 3 212 7660; stewartislandexperience.co.nz ) runs ferry services from Bluff to Oban three times daily.
* Port of Call (00 64 3 219 1394; portofcall.co.nz ). Double room from NZ$385 (£180) including breakfast.
* For details of walks, visit the visitor centre in Oban, or see doc.govt.nz/parks-and-recreation .
* Tourism New Zealand: newzealand.com