Bird watchers, nature-lovers - this slice of New Zealand's for you

When a New Zealander dreams of an island vacation, he's as likely to think of Great Britain or Manhattan as his own Stewart Island. Yet Stewart, situated 20 miles toward Antarctica in the Tasman Sea, has rugged charms that could be vastly appealing to non-New Zealanders - if they only knew about it. Most overseas visitors, however, rush through New Zealand after a longer tour of Australia and miss Stewart altogether, with its unspoiled treasure of mighty forests, mountains, moors, tussock plains, and splendid beaches.

Those who do come take a plane, train, or bus to Invercargill and then a ferry or plane to the island. The eight-hour bus ride from Christchurch has lovely rural scenes of six-foot thistle bushes, wagonwheel fences, windbreaks of flat-trimmed yew, harrow wheels of filigree iron like spinning Aztec suns, and sheep with turquoise ribbons in their ears. The bus driver makes spectacular tosses of the daily paper into the yards of remote farms and says ``aye'' and ``g'day'' as he stops to pick up crates of chicks and kiwi fruit. Where to stay

Traveling on my own, I chose the convenience of Gerrard's Railway Hotel, across the street from the Invercargill railway and bus station. Inexpensive hotel rooms in New Zealand are usually modest, but even $40 is too high for a tiny room, slow breakfast, and an all-night jam session of freight trains.

An option is to stay at the Invercargill Youth Hostel. A year's worldwide membership costs $25, and a night's lodging is $10. There's a large kitchen and common room where one can cook up local produce and visit with travelers from all over the world. Hostel hosts know where to stay in New Zealand and where there's temporary work to be had. A log containing details of treks on Stewart Island is studied and debated by those embarking the next day on their own walkabouts. The camaraderie of an Australian sing-along or a Japanese workers' dance certainly beats a lonely hotel room.

Next morning a taxi will take you to the H&H bus terminal for the ride to Bluff and the ferry to Stewart Island. The ferry ride takes two hours in good weather, and you can soon see the gray hills and silver mist of the island. If the weather is bad, Foveaux Strait can be one of the world's roughest stretches of water. You could fly, of course, but the adventure of reaching an island is in long views and crashing waves.

Besides, from the ferry you'll sight sooty shearwaters, fulmars, petrels, and albatrosses that you'd miss on the plane. And strangers traveling to an island are apt to talk to one another. Two American botanists from Washington State University told me of their plans to hike for a week, staying in huts and fishing for their suppers.

It's a good idea to make hotel reservations yourself or through New Zealand government tourist offices, which charge for the service. ``Good as gold'' means they've got you a booking, but ``chock-a-block'' means they're filled up. That's most likely to be the case during the school holidays, especially late December and all of January. February is a fine time to go to Stewart Island: It's the summer month with the least rainfall. Even the winter is mild, but the ferry may run only twice a week.

Resist being booked into the South Sea Licensed Hotel, because the rooms are in poor shape. Try for a room at the luxury Stewart Island Lodge, a housekeeping unit at the Rakiura Motel, a trailer at Ferndale Caravans, or a bed-and-breakfast in an islander's home. If you bring a tent to New Zealand, it should be provably unused, or it may spend valuable time in Customs being washed. I think the best place to stay is Horseshoe Haven Lodge, on its own two-mile curve of golden sand. It's 2 miles from Oban, the only town, but there is a courtesy coach, plus informal pickups by owners of the island's few vehicles.

At Horseshoe Haven, you can stay in a chalet for $55 to $130 a night for two, or in a dorm room for $22 each. The lodge is near hiking trails, and you can rent bikes, canoes, and dories. What to do

Once you're settled in, your first stop should be the Forest Service office in Oban. It has maps and current trail conditions and displays of island flora, fauna, and history. The people there will assist you in planning overnight hikes. Experienced trampers enjoy the full eight-day circle of the northern end of the island and insist it's far superior to the busier Milford Track. There are free overnight huts with running water, but you must carry all your food. Carrots, rice, and fishing hooks are recommended.

The trails are rugged, and there are swamps and sinkholes to be avoided, but the rewards can be seen in the exhilarated faces of muddy and exhausted hikers. They've observed crested and little blue penguins, kiwis even in daylight, and white-tailed Virginia deer. They've caught blue cod easily, using mussels for bait. They've found solitude and self-reliance while absorbing some of the most beautiful untouched scenery in the world.

Shorter hikes are at hand for those less experienced. You'll climb over stiles, go past deserted Hopper-like farmhouses, cross swing bridges, and descend through primeval ferny bush to uninhabited sandy beaches. A bus labeled ``Paradise'' makes tours of the 12 miles of paved roads.

The tourist office in Oban can arrange everything, including hunting, fishing, scuba, and launch trips. They'll fly you to remote parts of the 670-square-mile island and pick you up hours or days later. For $30, you can spend a long afternoon sailing to an outer island sanctuary, where the fantails, bellbirds, kingfishers and flightless green parrots take an interest in your arrival and itinerary. On the way back you'll catch enough blue cod for several meals.

At Horseshoe Bay, you can watch a Bellows painting come to life as scarlet seaweed is gathered with horse and wagon. The salt kills grubs in the garden, and the iodine nourishes the soil.

You can lie on a beach or stare down from a high bluff at the mollymauks seeming to walk on water, and the cormorants that sail regally by with long black necks, like sea snakes.

Yes, there is an island very like the one we dream of - where life is simple and nature triumphant. It is Stewart Island. If you go

To get to New Zealand, call Air New Zealand - telephone 800-262-1234.

The New Zealand Tourist Office (Suite 530, Rockefeller Center, 630 Fifth Ave., New York, NY 10111, [212] 698-4680) will provide Stewart Island information, railway and bus schedules, and details on hosteling.

A United States travel agent can book most of the following accommodations, except for the caravan parks and B&Bs; otherwise you can make arrangements through any tourist board when you arrive in New Zealand:

YHA hostel - 122 North Road, Waikiwi, Invercargill or Flynn's Hotel in Bluff.

Horseshoe Haven Lodge. Telephone: Stewart Island HMB 156K.

Stewart Island Lodge - rooms, all meals for $150 a night, per person. Telephone 021-391-085 or HMB 25.

Rakiura Motel - $66 for two people in a self-contained unit. Telephone HMB 27S.

South Sea Hotel - $40 per person a night. Telephone HMB6.

Ferndale Caravan Park - House trailers equipped with refrigerator and stove, $44 a night for two. Telephone 021-391-176, or HMB 52M.

Bed-and-breakfast with Mrs. J. Riksem, H. Slingsby, or Ann Pullen. Mrs. Pullen charges $7 a night for very basic bed, kitchen, and shower for hikers.