Kim waves her phone at the barcode-like QR square on the track marker. There are eight of us standing around watching, and she must be feeling the pressure.
She waves it again then stands up and says, "South Crater to Red Crater. Grade: moderate/difficult. Allow 1 hour".
Kim Manunui is Project Tongariro's media co-ordinator, and she's reading from a free smartphone application they've developed called the Pocket Ranger that can scan QR codes. We're standing, as the app says, in the South Crater on an Easter evening taking part in a moonlit crossing organised by Project Tongariro, and I'm there to learn more about the Pocket Ranger.
Project Tongariro is a community organisation established as a living memorial to five park rangers who died in a helicopter crash on Ruapehu in 1984. It is dedicated to educating people about the national park's natural history and runs conservation projects at Lake Rotopounamu, Waimarino wetlands and a kiwi recovery project called Operation Nest Egg.
I stumble along at the start of the crossing trying to walk and take notes from Kim. The Pocket Ranger was developed in partnership with the Department of Conservation in 2009 to provide tourist information on local accommodation, activities and transport.
Kim explains the 12 tiny QR codes mounted on the sides of prominent track markers provide unobtrusive interpretive information for each section of the track. Normal signage gets blown away in hurricane force winds and the Ngati Tuwharetoa, who gifted the park to New Zealand, object to ugly signs along the track.
Eventually DoC wants to roll out similar schemes across the country.
The 19.4km Tongariro Alpine Crossing is the best one-day trek in New Zealand. It's absolutely, stunningly beautiful. Its red, yellow and orange sulphur landscapes are a Martian world. No wonder it's so popular.
Our guide - Hakan Svensson, known as Hogi - reckons more than 2000 people are attempting the crossing over Easter weekend, and more than 100,000 visitors this year, many from overseas.
Hogi is a search and rescue volunteer, and has saved many people - including bejandalled boy scouts - from the crossing's notoriously harsh weather. But he doesn't criticise the overseas visitors as we huff up the new boardwalk detour that replaces the steep, scoria rubble of the Devil's Staircase.
"Ninety per cent of the rescues across New Zealand are Kiwis," he says. He says the new boardwalk we are on is part of a strategy to make the crossing easier to retreat from in bad weather, and therefore safer for everyone.
It cost DoC $1 million per kilometre to construct as all the materials had to be flown in by helicopter, but it will save on rescue costs in the long term. The Pocket Ranger plays a role in making the crossing a safer experience.
We walk across the flat, red expanse of the South Crater, then up to the high point of the crossing: Red Crater. The gash of the volcanic fumarole steams and glows red as we stand with the last of the sun streaming through the light mist. The colours are amazing and there's no one around.
No wonder people are increasingly doing the trip over winter to escape the crowds (including a Project Tongariro working winter trip this year). The moon rises and it's dark quickly. We put head torches on for our descent.
As we walk down through the moonlight I think about something Hogi said as we started, "You can see the movement here, I think that's my favourite thing about the valley."
He explained that it's one of the few places in the world where volcanoes and glaciers have come together to create the landscape we see today. And it's a living landscape. Lava flowed from Ngauruhoe as recently as 1954 and Ruapehu continues to burp and rumble.
The Pocket Ranger might not replace Hogi but it makes the crossing a richer experience.
Contact them for details of the upcoming winter trip (planned for weekend of September 1, depending on weather). Email firstname.lastname@example.org to register. Ph (07) 386 6499, tongariro.org.nz. The Crossing starts at Mangatepopo Rd, ends 7km off of SH47. The Project doesn't have cabins but can help visitors find accommodation.
Walking Places: 1 Mangatepopo Rd, Tongariro. Ph 027 308 9689.
Winter Tongariro Crossings: (May-September): 2+ people: $180-$210.
Mountain Heights Lodge: 4576 State Highway 4, National Park Village Ph (07) 892 2833.
The Schnapps Bar: Findlay St (just off SH4), National Park Village. Ph (07) 892 2788. Hearty pub grub.
The Station Cafe: Cnr Findlay and Station Rd, National Park. Ph (07) 892 2881. Dining in the old station.
Fraser Crichton was a guest of Walking Places and Mountain Heights.