Approaching the summit of Pyramid Peak, the simmering lake came into view, steaming like soup in a white bowl – although the sudden whiff of sulphur reminded us this was not a soup you should ever taste.

There’s no alpine scene in Aotearoa quite like Crater Lake. At 2530m it’s the highest lake of this size in the country (about 500m wide, and 100m deep). Cradled in the North Island’s highest mountain, among winter snow, the steaming lake offers a remarkable juxtaposition of hot and cold, alpine and thermal, mountain and lake.

Heated by the magma chamber beneath, Crater Lake’s temperature fluctuates between about 10°C and 60°C according to volcanic activity. The lake’s colour provides a rough indication of temperature: a smoky grey usually means higher, while it’s blue-grey during cooler periods. Tempting as a warm bath might be, the waters are highly acidic.

Our route had been from the east, up from Whangaehu Hut and the leading ridge of Pyramid Peak. However, the most straightforward approach is from Whakapapa Skifield, where moderate slopes lead up the Whakapapa to a saddle between Paretetaitonga and the Dome.

Don’t forget to check for warnings of recent volcanic activity before any trip. Ruapehu is a highly active volcano, and on occasions the Crater Lake releases lahars down the Whangaehu Valley – the last time in 2007.