There are only two places in the world that meet a record seven out of 10 in Unesco's World Heritage listing criteria.

The first is Mount Tai, one of five gargantuan mountains in China's Shandong Province. The second is the Tasmanian Wilderness World Heritage area, a chain of six national parks that covers one fifth of Australia's southern island state.

Tasmania's Parks & Wildlife Service describes the area as one of the world’s last true wilderness regions. And the wildest, largest and least-visited section is Southwest National Park, comprising 6,000sqkm of gold-green ranges, glacial lakes, untameable rivers, rainforests and moors in the southwest corner of the Apple Isle.

Cyclonic-force winds and lashing rains pummel the park for nearly nine months of the year. When shards of sunlight finally pierce the thick rolling clouds in December, it welcomes two very exclusive groups of visitors.

The first group, numbering around 1,000 each year, is made up of independent hikers who fly to the former tin-mining settlement of Melaleuca on Tasmania's south coast, the starting point of the South Coast Track. With mind-boggling mountain vistas, fast river crossings, cramp-inducing inclines and inclement weather, the 84km slog to the eastern hamlet of Cockle Creek is considered one of the world's greatest – and most challenging – wilderness treks. Hikers must be strong enough to carry large backpacks with a week's supply of clothing and food, as well as cooking equipment, tents, sleeping bags, first-aid kits and distress radio beacons.

The second group consists of the 100-odd people who join one of Roaring 40's eight annual kayaking trips, which run from early December to mid-March. One of only two ecotourism companies that can operate in the park, Roaring 40's intrepid expeditions range from three to seven days and traverse the waters of Port Davey Marine Reserve, the most remote and unvisited part of Southwest National Park.

This exclusivity might quickly be disappearing, though, as Tasmania's Parks & Wildlife Service looks to turn the entire Tasmanian Wilderness Heritage area, including Southwest National Park, into the new ecotourism capital of the world. Among the 37 potential tourism proposals, according to the most recent figures from the Sydney Morning Herald, government officials are considering developing a luxury guided walk along the South Coast Track, which would require the construction of five permanent huts and clearly defined helicopter landing sites, bringing in many thousands more visitors to Southwest National Park each year.

“There's also talk of upgrading the landing strip at Melaleuca into an instrumental airfield,” said Roaring 40’s expedition leader, Reg Grundy. “If that goes ahead, planes would be able to take off and land 12 months a year and it would make it economically feasible to build a bunch of eco-lodges here. Whether or not that happens, I think it's inevitable this place will change.”

I joined Roaring 40's on a three-day, 112km expedition through the Port Davey Marine Reserve to see the park before it does.

“How was the taxi ride in?” Grundy asked. He was referring to our knuckle-clenching flight from the capital Hobart, which hugged the jagged, soaring sea cliffs of Tasmania's south coast before landing at Melaleuca. A salt-of-the-earth Bear Grylls-type character with a thick Aussie drawl, Grundy made light out of everything – except safety.

“I won't sugar-coat it,” he said. “This is an expedition on the edge of the world. There is nothing here. If the weather turns, we need to be confident enough to survive in the wilderness in a long-term situation.”

From the airstrip, we carried our duffle bags through a field sewn with wildflowers to the bank of Melaleuca Creek, where a couple of two-person sea kayaks loaded with provisions awaited. After donning an ensemble of wind- and waterproof gear, we settled into our vessels and paddled into the wild.

We spent the morning heading north into Melaleuca Inlet, a narrow body of water lined with mangroves, stopping for lunch at Forrest Lagoon, a standing camp with a dozen bungalows 6km downriver from Melaleuca. After knocking back a few sandwiches, we paddled north into Bathurst Harbour before veering west into Bathurst Narrow.

It was here that we first saw Port Davey Marine Reserve's tea-coloured waters, stained by tannins created by the constant confluence of fresh and saltwater, and edged with quartzite beaches and rocks covered in fluorescent green or yellow moss. Port Davey is one of the most bizarrely coloured estuaries on Earth.

Beneath the surface lies an even more beguiling world – an ecosystem devoid of light that's home to sea fans, soft coral, biscuit starfish, orange sea urchins, tube worms and other delicate marine invertebrates found nowhere else on the planet. The low light and nutrient levels affect the entire breadth of the food chain; there are very few birds at Port Davey, rendering the reserve unnervingly quiet.

We spent the first night camping on Balmoral Beach, a quartz white sliver of sand dwarfed by the imposing crags of Mount Rugby. It was the height of summer but we still felt cold, tucked into our sleeping bags inside our steamy one-man tents. Time lost all meaning as we paddled further upstream into Bathurst Channel, the 12km-long waterway that connects the estuary to the Southern Ocean. We passed celery-top islets that haven't experienced fire for centuries and are crowded with tree and plant species long extinct on the shore.

The water was much deeper here – up to 40m in parts – inky black in colour and banked by rock walls covered in orange sea anemones, fluorescent green lettuce weed and yellowish bull kelp. Behind them in every direction rolled blue-grey mountains ranges, their dark eucalypt-lined gullies like folds in the hide of a sleeping giant.

On our second day, we made camp near the site of the former whaling station at Bramble Cove, where we found parts of irregularly shaped rum bottles hand blown in the early 19th Century. We paid our respects at the grave of Critchley Parker Junior, a Melbourne businessman who got lost and died from exposure in the early 1940s while surveying land during a hare-brained plot to establish a homeland for European Jews. We also visited one of 37 known Aboriginal cave sites in the Port Davey Marine Reserve, where archaeologists have found 30,000-year-old stone artefacts and evidence of shellfish exploitation.

The next morning, we broke camp an hour before dawn and made a beeline for the mouth of Bathurst Channel and Port Davey proper. Exposed to the South Ocean's currents, Port Davey’s shoreline is a mishmash of exposed sea cliffs, sheltered inlets, windswept beaches and islets honeycombed with paddle-through arches.

The first set of islets – the Breaksea Islands – lie less than 1km from the mouth of Bathurst Channel. But getting there proved challenging. Unlike the calm reflective waters of the estuary, the waves in Port Davey were 4m high – and can climb up to 12m during ferocious winter storms.

“It's a strange mix of terror and excitement when the first big waves sweep under your kayak,” Grundy quipped.

Upon reaching the islands, we spent an hour exploring sea caves, paddle-through arches and towering rock pinnacles as the restless sea rolled and crashed on a rocky, windswept coast. But Grundy pointed out we couldn't spend very long out here. The Southern Ocean is home to the some of the largest waves in the world and a slight change in the weather would have left us very vulnerable. Soon after we turned around and headed back into the estuary for the return leg of our journey.

The tannin-stained waters slipped quietly under our kayaks as we paddled back to Melaleuca. In this sacred part of the world where the elements still dominate, my sense of solitude was absolute. After working as a travel writer for 15 years, I've had the fortune of visiting some of the most exotic, unusual and remote places in the world. But few have compared to Southwest National Park, a timeless utopia on the very edge of the world.