Rocks and a 4-mile-high cloud of ash came out of the Te Mari craters but there was no lava flow, The (Wellington) Dominion Post reported Tuesday.
Nic Peet of the local Conservation Department said there were no deaths or injuries and the only damage was to a tramping hut near the crater. He said if anyone had been in the hut at the time, they could have been killed.
Four other huts on the mountain have been closed for safety reasons along with the Tongariro Alpine Crossing and Tongariro Northern Circuit, he said.
The eruption late Monday caused several small earthquakes.
Scientists said the mountain could have a major eruption again depending on what is causing the unrest.
"If it is steam driven, it's unlikely to do much more because once the initial pressure drops occurred you'll just get smaller activity," said consultant Brad Scott of New Zealand's GNS Science.