Lingering ash from a volcanic eruption in New Zealand disrupted flights across parts of the North Island on Thursday, as scientists warned the mountain could blow again at any time.

Mount Tongariro, in the centre of the North Island, erupted for the second time in four months on Wednesday after lying dormant for more than a century, spewing a column of ash three kilometres (1.9 miles) into the atmosphere.

Air New Zealand said flights to Rotorua resumed early on Thursday but services to Taupo and Gisborne would be subject to cancellations and delays until the ash cloud cleared, likely later in the day.

"The airline is working with the relevant authorities to safely make adjustments to flight routes to ensure aircraft remain clear of any ash and keep providing a safe service to the travelling public," it said.

The official monitoring body GNS Science said the volcano, which was used as a backdrop to Peter Jackson's hugely successful "The Lord of the Rings" films, was quiet overnight but remained unpredictable.

"We (can) expect another eruption of similar size at any time during the next few weeks," volcanologist Brad Scott said. "Eruptions are not expected to escalate in size."

Just days before the mountain rumbled to life, scientists warned that neighbouring Mount Ruapehu was ready to erupt as pressure builds in a subterranean vent.

They said there was no warning before the Tongariro blast and could not say if the activity at the two volcanoes was linked.

No injuries were reported from Wednesday's eruption but about 50 hikers scrambled for safety as ash billowed from the mountain.

With two volcanoes now active, the Department of Conservation said walking tracks and a number of roads in the Tongariro National Park had been closed until further notice.

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.

A 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 a river with the loss of 151 lives.