At 6:40 am on 10 December, 2009 a large pyroclastic flow travelled down Tyers Ghaut. This pyroclastic flow reached to below the west end of Lees village in the Dyers river, some 3.5 kilometers from the lava dome. This point has been stated by the MVO as the ‘Trigger point' for raising the hazard level from 3 to 4. It signifies that larger pyroclastic flows moving down the Belham valley are a more likely possibility. Larger pyroclastic flows could be formed by a partial dome collapse which could involve several million cubic metres of material.

Since 24 November an increase in volcanic activity has been associated with growth of the lava dome at the summit, and in the last two weeks growth has become focused on the northern side. This has lead to approximately 100 m of lateral growth of the lava dome in a northward direction. Increasing the available material for the formation of pyroclastic flows.

Pyroclastic flows down the northern flank have been abundant in the last few weeks, first occurring in Tuitts Ghaut. Over the last week these have started to move down Tyers Ghaut and the have increased in runout distance steadily over the past few days.

Following a meeting of NDPRAC which concluded at 4:30 pm, the decision was reached to raise the hazard level to 4.