WELLINGTON, New Zealand (AP) _ Blasting out rocks as big as cars and belching ash and steam 12 miles high, Mount Ruapehu could produce New Zealand's biggest volcanic eruption in 50 years, scientists said Monday.

Authorities diverted air traffic, canceled train service past the volcano, and closed the highway around its base and the ski fields on its slopes.

Eruptions from the Crater Lake area were occurring every two to three minutes Monday, with ash, rock and steam shooting out of the 9,000-foot peak, which is midway between Auckland and the capital, Wellington.

Civil Defense officials put the region on a ``level four alert,'' which means a large-scale eruption may be near. They warned people living within 60 miles downwind to expect heavy ash falls.

Rescue services at the Waiouru military base, 12 miles from the mountain, were placed on full alert, and wives and children at the base were evacuated.

There were no widespread evacuations, however, not even of the 60 residents of Whakapapa Village on the volcano's slope.

`It looks like thousands of rockets going up,'' one man said, standing outside Whakapapa Visitors Center where dozens gathered to watch the show. Seconds later, more ash and rock spewed high into the blue sky.

The few remaining ski staff partied Monday at Trails Tavern in Whakapapa Village, cheering each new explosion. The village has an alarm system to give 20 minutes warning if a mud flow from the slopes heads towards town, giving residents time to reach high ground.

Ian Nairn, a volcanologist at the Institute of Geological and Nuclear Sciences, said the mountain's volcanic activity was the most sustained since 1945, when ash ruined crops, contaminated water and made grass unpalatable to livestock.

Another volcano expert, Alan Smith, said Mount Ruapehu is ``blowing its lake out. It's clearing its throat.''

People living within 18 miles who depend on wells, streams or dams for water have been advised to turn off their water to avoid pollution from toxic ash, said Civil Defense director Paul Officer.

The Civil Aviation Authority closed the air space over much of the central North Island, concerned that volcanic ash could clog turboprop airplane engines. Two airports _ at Gisborne and Napier _ were closed after reports of ash falling on the runways.

Falling ash settling on power lines was causing power surges throughout the North Island, making lights flicker, the utility Trans Power said.

Overnight passenger train service between Auckland and Wellington, which runs past the west side of the mountain, also was canceled. New Zealand Rail Ltd. officials were concerned tracks would be washed out by mud flows or drifted over by ash.

In 1953, a mud slide caused by the collapse of the rim of Crater Lake washed out a railway bridge. Minutes later, the Christmas Eve Wellington-to-Auckland Express train plunged into the river, killing 151 people. It was New Zealand's worst railroad disaster.

State Highway 49, the road that runs around the base of the mountain, also was closed Monday night.

On Sunday, four scientists photographing volcanic activity were injured along with the pilot when their small plane was forced to make a crash landing due to engine problems.