"See that hole," says Ross, pointing at a steaming chasm in the rock face a few metres away. "That wasn't there yesterday. Which is why we don't walk that way."

"That way" is a bubbling maze of mud pools and steam vents – an alien, sherbet yellow landscape that's gurgling and fidgeting like it's alive. Which, in a way, it is. We're standing in the crater of White Island, one of New Zealand's most active volcanoes, and, frankly, I'm astonished we're allowed to be here at all.

Ross, our pilot and guide from Frontier Helicopters, assures us he checked the volcanic activity on the island's seismic sensors before we left. On a scale of one to five, it's currently at one, indicating a low level of background activity. Once it gets to three, they're no longer allowed to visit.

MATT BOWEN/FAIRFAX NZ White smoke emerges from White Island.

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Rob McFarland Landing in the crater on White Island.

It takes us precisely 11 minutes to cover the 48 kilometres from Frontier's base in Whakatane on the east coast of the North Island to White Island. At first all we can see is a towering plume of steam on the horizon. It's a similar view to the one Captain Cook would have had in 1769, except he mistook the steam for smoke from Maori fires. Not wishing to get into an altercation, he gave it a wide birth but named it White Island.

Today, the island is privately owned by the Buttle family after a canny ancestor allegedly purchased it from the local Maori for two barrels of rum.

From a distance the island looks like a solid dome of rock but as we draw closer it's revealed to be a dramatic crater roughly 2km in diameter. The steep-sided rim rises 321m above sea level, but that's only the top 12 per cent. There's another 1280m beneath the surface, a vast cone that measures 16km by 18km at the sea floor.

Rob McFarland Mud pools on White Island.

After a scenic fly-by we land on the crater floor and tentatively get out to explore. Equipped with hard hats and gas masks ("for insurance purposes only," says Ross), we follow him as he carefully picks his way across the belching terrain.

The surface is littered with scoria, a light, honeycombed volcanic rock in a kaleidoscope of colours – from rust red to lemon yellow to bottle green. Many areas are covered in a thin white crust of calcium sulphate indicating there's scalding gas trapped beneath the surface.

We cross a small stream and Ross encourages us to try the water. It tastes disturbingly like blood – warm, salty and metallic thanks to the high iron content.

Students study geography at White Island.

Continuing towards the source of the billowing steam, we find ourselves on the edge of a large crater filled with water. Occasionally the steam clears and we catch a glimpse of its murky green surface. The water has a pH of around 0.3, making it more corrosive than battery acid. Ross says it's one of the most acidic crater lakes in the world.

We pause for a minute, the pungent, acid-infused steam catching the back of our throats, and listen to the steady roar coming from the other side of the lake. It sounds uncannily like a plane, but is actually the ominous grumbling of the volcano beneath us.

Unsurprisingly, wildlife is sparse. There's a colony of Australasian gannets on the southern edge of the island but other than that it's just flies, flax and a few hardy ice plants.

Picasa Scientists take samples of the hot springs on White Island.

We walk towards the shore and explore the corroded remains of a sulphur mining factory that operated in the early 1900s. The sulphur was extracted from rocks and sent back to the mainland to be used in fertiliser, explosives and medicine. Before the days of antibiotics, it was used as an antibacterial agent and Ross has had guests who can remember buying bags of it from the pharmacy.

Mining eventually ceased after part of the western rim collapsed in September 1914. The resulting landslide killed 10 miners; only the camp cat survived.

Despite visiting more than 700 times, Ross says every trip is different. He recalls bringing a fellow pilot back after eight years who said that the yellow vents we examined earlier used to be hundreds of metres away on the other side of the crater.

It's difficult to imagine such dramatic transformations in such a short time span. But then I watch a YouTube video of the island's most recent eruption in 2013. Thankfully no one was there but it's powerful proof of why Maori call this place "Te Puia o Whakaari" – the dramatic volcano.

TRIP NOTES

MORE INFORMATION

bayofplentynz.com

GETTING THERE

Emirates flies from Sydney and Melbourne to Auckland. Phone 1300 303 777, see emirates.com.

TOURING THERE

Frontier Helicopters' two-hour White Island Volcano Adventure costs $650 per person. See whiteislandvolcano.co.nz.

Rob McFarland was a guest of Frontier Helicopters.

Traveller.com.au