The Department of Conservation is installing warning signs after up to 30 people ventured across the frozen Hooker Lake during the weekend.
The glacial lake is located in Aoraki/Mount Cook National Park in Canterbury, part of the Hooker Valley Track.
During the weekend, between 20 and 30 people were photographed exploring the frozen waters.
That's not a good idea, according to the Department of Conservation.
"It is not safe for the public to walk across the frozen glacial lakes and DOC strongly recommends people do not attempt this," Aoraki/Mount Cook operations manager Sally Jones told 1 NEWS today.
"The ice is always weak in the centre of the lake and where the lake connects with the Hooker River."
If the ice cracked, it would send the person plummeting into the frigid glacial waters. Hypothermia would quickly develop.
"Our message is clear. Please don't walk on the lake if it's frozen, this is dangerous and people could easily fall in," Ms Jones says.
If someone fell in, and others tried to help, Ms Jones says they'd be putting their own life at risk.
The frozen lake can be used to access parts of the national park that are otherwise hard to reach.
Only experienced climbers should consider crossing the frozen lakes, Ms Jones says, after assessing the ice thickness.
"Experienced climbers are trained to rescue themselves if they fall through ice, but the general public won't have the right skills to undertake a successful self-rescue."
One of the people who traversed the frozen lake was adventurer James, who says he's been there seven or eight times.
He estimates the ice was around four to six inches thick, thick enough to hold his weight.
"The ice freezing that thick clearly doesn’t happen particularly regularly and was a rare experience," he says.
"After looking at the thickness of the ice and making my own judgement I deemed it safe."
However James isn't encouraging others to leap in and do the same, warning, "If in doubt, don't do it."
DOC is planning to install a warning sign near the lake.