Trampers caught by freezing cold on Tongariro Alpine Crossing

Trampers have been warned to prepare for freezing cold conditions on the Tongariro Alpine Crossing in the central North Island.

Temperatures plunged to sub-zero levels overnight, prompting cautions from alpine safety experts.

Those who braved the weather endured temperatures as low as -13degC and many were caught unprepared.

"It was really cold up there and I didn't have any gloves," one tramper told Newshub.

Another said, "We came from Waiheke and turned up in the gear we came with, and Converses are definitely not the right option."

Ruapehu Scenic Shuttle owner and driver Colin Baker was so concerned about the strong winds, he turned down a group of trampers who arrived late.

"With today's conditions, I just felt it was inappropriate to take any more people to the crossing."

Local tour guides say it is essential to be prepared with plenty of warm clothing even in the height of summer, because it can still snow.

In fact, nearly half of all mountain deaths happen in December and January. The central North Island is the most risky region, with 10 times as many people needing rescue as the rest of the country.

And the higher you go, the greater the danger.

"Every 150 metres you climb up, you lose one degree Celsius, so if you're climbing 1000 meters you're going to drop seven degrees," says Adrift Guided Outdoor Adventures director Stewart Barclay.

The MetService says it is unlikely there'll be any more snow but temperatures are still well below 0degC.