Mount Tongariro, one of three active volcanoes on New Zealand's North Island, erupted on Wednesday and a helicopter was sent up to look for hikers on a popular walking route, dpa reported.
The 1,968-metre mountain, which erupted on August 6 for the first time in 116 years, sent a plume of ash 2 kilometres into the air and civil aviation authorities issued a warning to aircraft.
There were no reports of lava or rocks threatening populated areas.
Light winds meant the plume went straight up and dissipated, but Civil Defence officials said light volcanic ash could fall over four North Island provinces and people in affected regions should stay indoors.
A helicopter pilot who witnessed the eruption while flying over nearby Mount Ruapehu told Radio New Zealand he saw up to 20 people near a hiking hut on Tongariro's northern slopes.
The Department of Conservation sent up a helicopter to look for hikers on the 19.4-kilometre Tongariro Alpine Crossing, a popular trail used by thousands of walkers every year.
The state GNS Science organization, which monitors earthquakes and volcanoes, announced earlier this month that pressure was building under the Crater Lake at the summit of Mount Ruapehu (2,797-metres) and there was an increased likelihood of it erupting.