As I glide up to a narrow gap between two walls of rock, white water spits angrily out of the chasm ahead of me. It’s only just wide enough for my boat to enter. Sunlight skitters across the ripples and I take a deep breath. “You need to paddle hard and fast to get through,” says my guide, Arno. “If you capsize, ditch your boat and swim out as quickly as possible.”

I go for it and my biceps burn with the effort of each stroke until, at last, I come swerving into a canyon of turquoise water, surrounded by high, mossy cliffs and twisted beech trees. The only sound comes from the waterfall ahead, spilling its glacial meltwater around us.

This secret corner of the Rees Valley feels a world away from the bungee-jumping tourist bubble of Queenstown – yet we’re only an hour’s drive north.

Arno tells me that 99.9% of people who’ve been in this valley haven’t seen this spot, “because you can’t get here without a boat”. Even getting here with a boat has been tricky for us. I carried mine here on my back, to try out the growing sport of packrafting – trekking into the wilderness with a small, stowable rubber craft in your rucksack.

I’m not the only one: the American Packrafting Association, established in 2012, has more than 1,000 members in 30 countries, and estimates that 10,000 people are now packrafting worldwide. Alpacka Raft, the largest packraft firm, started selling boats in 2001, and has seen growing interest and sales ever since.

Packrafting also has a growing presence online, with blogs including Things to Luc At, created by Luc Mehl from Alaska, who says: “I grew up on a river, but I never thought of it as a trail until I got a packraft. They open tons of new terrain for adventuring.”

The kit doesn’t come cheap – a basic Alpacka Raft costs £612 – so I’m trying a new two-day tour with Expedition X, New Zealand’s only packraft tour operator. For the truly hardcore tThey also offer a hardcore six-day expedition into the Fiordland national park that involves boulder hopping, coastal walking and paddling across rivers, lakes and sea. (You don’t have to travel this far: Back Country Biking runs mountain biking and packrafting trips in Scotland.)

We begin at Muddy Creek, near the sleepy town of Glenorchy, with kit packing and a visit to the last loo we’ll see for two days. “I hope you’re feeling adventurous,” Arno smiles.

The aim is to spend the first day hiking (or tramping as Kiwis call it) 13km into the bush, set up camp, then use our packrafts to return by river. My backpack holds 20kg of tent, stove, food, warm clothes, wetsuit and my secret weapon: folded into a pillow-sized bag and weighing just over 2kg, my packraft.

Inflatable rafts were developed in the second world war, for use at sea by downed pilots. They were adopted by wilderness travellers in Alaska in the early 1980s. Modern versions, like the one I’m using, can negotiate the kind of whitewater usually reserved for specialist kayaks. Mine has a paddle that comes apart into two pieces, making it easy to stash in a backpack.

Our group of five consists of Charlie, a personal trainer from Queenstown, my boyfriend Chris, me, and guides Arno and Huw. Expedition X has two guides for safety reasons on its multi-day trips - and we’re in safe hands. While Arno was in the German special forces for a decade, Huw, from north London, has led expeditions everywhere from Everest to the Inca Trail.

Beneath a cloudless sky, we set off on foot, skirting the Rees river through golden grasslands backed by trees and the snow-covered peaks of the Richardson and Forbes mountains.

The Rees Valley is named after William Rees, a Welsh sheep farmer and Queenstown’s founder, who came looking for farmland before the 19th-century Otagao Gold Rush that attracted a deluge of prospectors before all the gold ran out.

As we cover clumpy, sometimes boggy, terrain, Huw points out Lennox Falls, plunging over the Forbes mountains, and Mount Earnslaw, whose meltwater runs into the Rees: “They say no man ever steps in the same river twice and that’s never been truer than for the Rees.”

He explains how the loose riverbed gets rolled around after flooding, so the river’s path can change in as little as 24 hours. “Rivers don’t just sit there like a mountain; they are constantly moving, so we need to adapt.”

Soon we’re making the first of many river crossings, wading through fast-flowing, knee-high water that makes my skin tingle in spite of my neoprene socks. Arno and Huw teach us how to cross safely, on a diagonal, using small steps, and my confidence grows.

At lunch, we eat sandwiches with the sun warming our soggy feet and only cows for company. By 4pm we reach a clearing and set up camp, gathering firewood and water from the river. Within an hour we’re watching the flames lick and snap around our stove. Arno pulls out a map, tracing his finger along our trail so far: “Hikers used to view these rivers as a barrier, but with packrafts, rivers and lakes become part of the journey and open up new places.”

As darkness falls, we eat pouches of pasta and toast marshmallows, while counting shooting stars. My shoulders and hips ache from the weight of my pack and I go to bed thankful that I won’t be carrying it again tomorrow.

The following morning, we unroll our packrafts. They come with a clever but simple pump system – a large bag with a nozzle on one end. Arno shows me how to screw the nozzle into the boat and waft the opening to catch a large bagful of air. Then he folds the top over, shoves the inflated bag under his arm and squeezes it into the boat, repeating until it is inflated. It needs only a quick top-up from lung power to make it rock-hard.

Proficient packrafters can have their boats inflated in around 60 seconds, but my first attempt takes 10 minutes. Then we enter the water. The one-man rafts are surprisingly stable and easy to control, even with a hefty backpack strapped over the front.

We learn to use eddies to stop on the banks, and how to spot deeper water by its darker colour, and “wave trains” – small ripples of white water.

In a packraft, without the distraction of a heavy backpack and tired feet, you can lie back and simply feel a place as it drifts by. I see it with fresh eyes. We follow Arno like brightly coloured ducklings chasing after their mother. Occasionally we have to carry our boats around fallen wood or shallow sections – but the water quickly gathers momentum, and so do we. Panic turns to pleasure as I bounce over splashing rapids, adrenaline racing. And it’s all the more thrilling because we’re seeing parts of New Zealand most tourists don’t get to see.

• The trip was provided by Tourism New Zealand (newzealand.com). An overnight packrafting trip with Expedition X (packraftingnz.com) costs from NZ$799 (around £343). One-day trips cost from £137 and half-day trips from £77. Future overnight trips will be held in other locations, are entirely bespoke and do not feature on the company’s website – email for details. Flights were provided by Air New Zealand, which flies from Heathrow to Queenstown, via LA and Auckland, from £938 return