GNS Science has raised the volcanic alert level of Mount Ruapehu to Level Two, which stands for "moderate to heightened volcanic unrest." Level Three indicates a small eruption.
"We have many episodes of unrest but very few eruptions," Brad Scott, a volcanologist at GNS Science, told CNN.
Hikers are being told to keep at least two kilometers away from the crater's center.
"We recommend climbers, trampers and walkers (not to) enter the zone" said Paul Carr, an operations manager at Tongariro National Park, in a press release.
"Guiding companies should also heed the advice and not take people into the zone."
Scott said GNS Science monitors seismic activity, volcanic gas output and heat flow to determine the alert level.
Temperature is rising
In the past two and a half weeks, the temperature of the crater lake at Mount Ruapehu has increased by 25 degrees, from 20 to 45 degrees Celsius (68 to 113 degrees Fahrenheit).
Scott said there have been eight heating episodes at Mount Ruapehu since 2010, but none of them resulted in eruptions.
Harry Keys, Technical Adviser of Volcanology for the Department of Conservation, said the alerts usually don't last long.
'Lord of Rings' fame
Tongariro National Park gained international fame as the film location for Mordor, the stronghold of the dark Lord Suaron, in the "Lord of the Rings" trilogy directed by Peter Jackson.
The series, based on J.R.R. Tolkien's fictional universe of Middle Earth, took the world on a tour of New Zealand's lush and barren landscapes.
Film locations included the Tukino ski field on Mount Ruapehu, which doubled as the Black Gate of Mordor.
None of the ski areas or roads on Mount Ruapehu and the rest of the park were affected by the volcano warning and were safe to visit, scientists said.