MT Ruapehu (N Island New Zaland) has had quite a few eruptions, and has been quite active. Over recent years, it has been relatively quiescent, with the crater lake bubbling over a few times only in the past twenty years or so.
The trouble with Mt Ruapehu id that the crater is above the winter snowline, and if it has even a minor eruption when it is covered in snow, the melted snow can cause a mud slide (called a lahar), which can rapidly move down the slope. The problem is that the slopes are also a very popular ski field in winter. The last major eruption was about twenty years ago, when a mud slide ,flowed down right through the middle of the ski area. Fortunately, this happened after the close of the ski season. There have been a few minor eruptions since; but no mud flows.
The most infamous of its eruptions was in the 1950's, when it erupted and melted the snow cap. The melt water rapidly flowed down the slope, swelling the streams to such an extent that it washed the main railway line away at Tangiwai. As fate would have it; the Auckland to Wellington express passed by a few minutes later and was swept away, many people being drowned. The Tangiwai disaster is still New Zealand's most severe natural disaster; and brought the hazard of lahars into international focus..
Last time I skied there about ten years ago, the crater was bubbling, and the ski fields were evacuated a week or so later, when there was a minor eruption.