The hot, empty, difficult bits that few Australians ever bother to visit - even in the era of air-conditioning. The Tropic of Capricorn cuts a neat line across the continent from the Indian Ocean to the Pacific, sectioning off about 80 per cent of Northern Territory, and the northern halves of Western Australia and Queensland. The latter state includes fast-growing Cairns and one of the wonders of the world, the Great Barrier Reef, so it's not all inaccessible wilderness, by any means.
AND THE WEATHER? HOT AND HUMID LIKE THE TROPICS?
In some parts, but not all. Northern Queensland has attractive tangles of rainforest, but most of the region is infernally hot and dry in the Southern summer (Northern winter), especially in the interior, where temperatures have been known to exceed 50C (122F) and droughts to last for a decade or more. The winters tend to be wet and squally, but are still hot and humid by European standards. On the north coast, the daytime temperature - summer or winter - is never less than 30C (86F). Here, the year is divided into just two seasons: Wet and Dry.
The best time to go is during the European summer, when it's bearable during the day and at night, if the sky's clear, the thermometer can dip below zero. It's enough to make you feel homesick.
In an area as vast as this (about the size of Europe) they're too numerous to list, but the clear market leader lies a few miles offshore in the Coral Sea, mostly beneath the waves. At 1,200 miles in length, the Great Barrier Reef, an ever-growing rampart of living and dead sea creatures, is the world's largest structure created by living organisms - a rare example of man playing second fiddle to the otherwise unsung coral polyp. You don't need to be a scuba diver to appreciate the incredible forms and colours of the coral, and the rich marine life that has made it home. Even from a glass-bottomed boat you might catch sight of a giant clam that could be more than a century old, mermaid-like sea cows, green turtles, manta rays or humpback whales.
Cairns is the best base for visiting the reef, although Townsville, the most populous town (150,000) in the Australian Tropics, is the home of Reef HQ (00 617 4750 0800; open 9am-5pm every day, admission A$20 (£8.30)), which has a wonderful simulation for determined landlubbers. The real thing is two hours away. Townsville's visitor centre at Flinders Mall (00 617 4721 3660; www.townsvilleonline.com.au) can provide information on any number of outdoor activities such as the superb whitewater rafting in the mountains between Townsville and Cairns.
Although it's the smaller of the two cities, Cairns is more tourist-orientated, with the fourth-busiest airport in Australia and innumerable nautical excursions to choose from. For instance, Great Adventures (00 617 1800 079 080; www.greatadventures.com.au) offers trips for about A$100 (£40) to several points of the reef in either a semi-submersible or glass-bottomed boat. The town has plenty of accommodation to suit all pockets, and a colourful and noisy night-life.
Cairns also offers easy access to a host of special places, notably the beautiful village of Kuranda, deep in the rainforest. You can travel hair-raisingly on a scenic railway line in a traditional wooden carriage, or swing gently up and over the foliage in a gondola on the Skyrail Rainforest Cableway (00 617 4038 1555; www.skyrail.com.au; 8.15am-5.15pm every day except Christmas.) You can go up in the train and return on the cableway for A$70 (£30). Whichever conveyance you choose, it's one of Australia's classic day-trips.
About 60 miles north of Cairns, Port Douglas used to be a sleepy seaside town, but now it is a glitzy resort full of international hotels and casinos. The visitor centre has a humdrum phone number (00 617 4099 3235) but one of the web's most exotic addresses: www.reefandrainforest.com.au.
In the far north of Queensland, the Atherton Tableland is a scenic plateau of lakes, waterfalls and forests, with the edge taken off the heat by its height.
ANY OTHER OFFSHORE HIGHLIGHTS?
Unrelated to the reef, Magnetic Island is an unsung gem, a 20-minute boat trip from Townsville. There are great beaches and walking trails, and it's mercifully empty. Much more remote are the Torres Strait Islands: 70 dots in the ocean from the northern tip of Queensland to within four miles of Papua New Guinea. A daily Qantas flight connects Cairns with the former pearling centre, Thursday Island, but it's pricey: A$830 (£350) return. An inexpensive alternative is the hour-long ferry crossing from the small mainland port of Seisia (A$75 (£30) return), but getting as far as Seisia is an endurance feat in itself.
CENTRAL AUSTRALIA: JUST A LUMP OF ROCK?
Far from it. The trouble is that pretty well everywhere in the middle is far from everywhere else. Uluru, to give Ayers Rock its Aboriginal name, is actually a little south of the Tropic of Capricorn, but this wonderfully weird place feels like the centre of the earth.
The indigenous community runs a number of excursions, such as the half-day trip costing A$110 (£45)) with Anangu Tours (00 618 8956 2123; www.anangutours.com.au), telling the story of the rock and the tribes who have lived in its shadow for thousands of years. Over a mere 20 years, the purpose-built resort of Yulara, 12 miles away, has kept pace with the growing numbers of tourists who head to the rock from all points of the compass. But it's a pricey place, with the cheapest rooms in the Outback Pioneer Hotel (00 618 8957 7605; bookings website www.voyages.com.au) costing A$185 (£77). If your time is limited, and money's no object, there's a breathless dawn-to-dusk round-trip excursion to Uluru from Alice Springs. It's organised by Day Tours (00 618 8953 4664) and costs A$1,200 (£500).
Most rock visitors pass through the Alice, as it's known, with its air connections to every major city, and a stop on the railway line between from the top end of Australia and the bottom.
NO LONGER A RAILWAY TERMINUS, THEN
No, this year the Ghan line was extended north from Alice Springs to the northern port of Darwin. The weekly rail journey on the new line takes 24 hours; details from Rail Australia (01572 768022; www.railaustralia.com). Darwin's young, lively, multicultural community makes up for any architectural shortcomings, and it is an excellent base for excursions into the Outback with companies like Aussie Overlanders (00 613 0088 0118; www.aussieoverlanders.com).
WHAT'S THE OUTBACK LIKE?
Indescribably vast and barren. To most Australians, let alone foreigners, the terrain is both daunting and dangerous, but the modern crop of adventure-seekers finds the prospect of these huge spaces on the map irresistibly challenging.
Signs of life crop up here and there. Two hours east of Darwin, in the beautiful savannah of Kakadu National Park, there are 5,000 ancient and modern Aboriginal paintings scattered about, especially on the ledges of the massive escarpment that brings the plain abruptly to a halt. Among the park's abundant wildlife are man-eating saltwater crocodiles. A seven-day pass to the park costs A$16.30 (£6.80). The Bowali Visitors Centre (00 618 8938 1121; www.marrawuddi.com) near the township of Jabiru, explains the natural forces that brought Kakadu into being.
WHICH LEAVES NORTH-WEST AUSTRALIA
In many ways, the tropical slice of Western Australia is the most god-forsaken part of all. The names of many geographical features reveal all you need to know: the Great Sandy Desert; Lake Disappointment. Not everyone has been disappointed, though. In the early 20th century, the lush, sleepy port of Broome, lying on the Indian Ocean only 18 degrees south of the Equator, was the world's leading producer of mother-of-pearl, while the mountainous Kimberley region, remote even by Australian standards, has yielded some exquisite diamonds, with the promise of many more to come. But they'll take a bit of digging out: Kimberley is one of the emptiest places on earth, its natural features and climate downright hostile to man. It contains a huge meteorite crater (at Wolfe Creek), but the region was so inaccessible that it was known only to the Aborigines until 1947 - and so became one of the last places on earth to be mapped.
Today, Broome relives its former glories, has an authentic Chinatown district, and, in Cable Beach, boasts one of Australia's most beautiful stretches of white sand.
HOW DO I REACH TROPICAL AUSTRALIA?
A good airline is Royal Brunei, which has flights from Heathrow via Bandar Seri Begawan to Darwin, Perth and Brisbane. Cathay Pacific serves Cairns via Hong Kong. Or get a cheap flight to Perth on, for example, Qantas or Singapore Airlines and take advantage of low-cost flights within Australia.