Vulcanologists were watching the 'Mount Doom' volcano, Mount Ruapehu, when Mount Tongariro exploded.
A New Zealand volcano used as a backdrop to "The Lord of the Rings" films erupted on Wednesday, spewing a column of ash three kilometers (1.9 miles) above the North Island, scientists said.
Vulcanologists said Mount Tongariro, in the center of the North Island, rumbled to life at 1:25 pm (0025 GMT), in a five-minute burst that briefly closed roads, put aviation authorities on high alert and sent nearby hikers scrambling for safety.
"It was completely unexpected, there were no warning signs beforehand," a spokeswoman for official monitoring service GNS Science told AFP. "We were watching (neighboring volcano) Ruapehu waiting for an eruption and instead this came out of nowhere."
Mount Tongariro was dormant for more than a century until August this year, when a massive blast ripped a new vent in its side and hurled boulders the size of cars more than two kilometers.
While officials said the latest eruption was minor by comparison, they still closed a number of roads for a time and warned aircraft to stay away from the area as a precaution.
About 50 people, including schoolchildren, were hiking in the Tongariro National Park when the eruption occurred but police said they were not in danger and there were no injuries.
Park guide Stuart Barclay said there was a sense of "not quite panic, but getting there" among students as they heard a loud bang and saw a billowing ash cloud rise before them.
"We got them out of there quick-smart," he told national radio.
The national park, which was used to depict the desolate wasteland of Mordor in Peter Jackson's hugely successful "The Lord of the Rings" movies, attracts 800,000 visitors a year to its ski-fields and hiking tracks.
Its three volcanoes do not directly threaten population centers, with the nearest town Taupo some 80 kilometers (49 miles) away, but they have proved deadly in the past.
An eruption at Mount Ruapehu in 1953 caused New Zealand's worst rail disaster when it triggered a massive mudslide that washed away a bridge, causing a passenger train to plunge into the Whangaehu River with the loss of 151 lives.
Further north from the park, Mount Tarawera erupted in 1886 with a death toll estimated at 120-150.
New Zealand lies on the so-called "Pacific Ring of Fire", where the Earth's tectonic plates collide, making it a hotspot for volcanic and earthquake activity. Local resident Clint Green said the latest eruption was "pretty spectacular".
"All of a sudden a towering black plume just began erupting very quickly, skyrocketing up," he told Radio New Zealand. "At first I didn't believe what I was seeing."
Airline services suffered only minor disruptions and all roads were open again by Wednesday evening. But officials said the park around the volcano would remain closed for at least five days in case of further eruptions.
"Unlike August, there were no ballistics (flying rocks)," the GNS spokeswoman said.
"This was basically just ash being expelled into the atmosphere, but we don't know what could happen next."
Locals were told they could expect light ashfall over the next few days and were advised to stay indoors with windows and doors sealed.
Scientists warned about increased volcanic activity in the area this week, saying that neighboring Mount Ruapehu is in danger of erupting as pressure built in a subterranean vent.
GNS said it was impossible to know if the rumblings at Mount Ruapehu and the surprise eruption at Mount Tongariro were linked.