Haast River Safari, South Island, New Zealand: Jetboat hijinks in a World Heritage Site

"As the old man would say, we're off like a dog shot in the arse," says Lindsay Kain in classic West-Coast-South-Island-New-Zealand-clipped-drawl. He hits the throttle and the jet boat glides upstream with the negligible judderings of a powerful animal running full pelt. This is Haast River Safari; a backcountry journey into the heart of Westland and the soul of its "Coasters".

The town of Haast and its river and pass have, like many places in New Zealand, been most recently named after a European man – in this case German geologist Julius von Haast. The 100-kilometre river runs east to west through Te Tai-poutini, or the West Coast region, and drains into the Tasman Sea. Despite decades of mining, forestry and farming, significant tracts of native forest remain and in 1990 the area was included in a UNESCO World Heritage Site.

Haast River Safari's fully enclosed all-weather vessels suggest soft adventure but Lindsay counteracts that with extreme stories about his father, who was a "farmer so he didn't kiss the kids", removing lamb testicles with his teeth on castration days and coming home with a ring of dried blood around his mouth. Forty-something Lindsay Kain, however, is a different generation and points to where he helped his daughter reintroduce her pet possum, Honey, to the wild which the family has since referred to as Honey Flats.

Although there are rapids closer to its source, the Haast is wide and gravelly and even a little braided where we are. One section has recently shifted its course to the opposite side of the valley and washed an unsuspecting bank away so 1000-year-old rainforest growth now sloughs into the river. This is not something Lindsay attributes to climate change, just nature doing its thing.

There are some pulse-raising moments as we skirt the gravel bars and occasionally rap against the shallow bottom but Lindsay knows this river of his childhood like the back of his well-tanned hand. The hour-long journey, up to where Roaring Billy Falls of Mt Aspiring National Park tumbles into the Haast, disappears in a splash. At the base of the 30-metre falls the deep water is strikingly blue from suspended Southern Alps glacial till.

Those not up for 360s then step off onto the pebbly bank. As an indicator of the potential power of Haast Safari boats, the river's rate of fall over the 30 kilometres or so between Roaring Billy and Haast Bridge, where we began, is 70 metres. Under full power the boat's two jet units move 600 litres of water per second. When someone on board jokes about spraying those on shore as we whip into a turn, Lindsay tells us we could do more than wet them. We could knock them off their feet.

After the first few spins he checks in with his passengers. "Lumps in the pants I don't mind because you can take them with you," he tells us, but vomiting he minds. Everyone's intact so we go around a few more times.