This information was published in 1966 in An Encyclopaedia of New Zealand, edited by A. H. McLintock. It has not been corrected and will not be updated.
Up-to-date information can be found elsewhere in Te Ara.
Maori Legend of Mounts Ruapehu and Taranaki (Egmont)
Ruapehu, the beautiful maid, was married to Taranaki. One day, while her husband was away hunting, she was wooed and won by Tongariro. When Taranaki returned at the end of the day he surprised the guilty pair. A titanic battle ensued in which Taranaki was defeated. He retreated towards the west coast, carving out the course of the Wanganui River as he went. When he reached the coast he moved northwards to the western extremity of the North Island, where he rested. There his great weight made the shallow depression which afterwards filled with water and became Te Ngaere swamp. Taranaki, or Egmont, as Cook named him, now sits in silence looking towards his wife and his rival. In spite of her infidelity, Ruapehu still loves her husband and sighs occasionally as she remembers him, while the mist, which drifts eastward from his head, is the visible sign of Taranaki's love for her. For his part, Tongariro, who despairs of ever possessing her again, smokes and smoulders with anger. To this day travellers in the Tongariro National Park see the basin called Rua Taranaki, “the Pit of Taranaki”, which lies to the east of the Tama Saddle which was the original home of Taranaki.
The name Ruapehu does not appear to commemorate any event in Maori legend. Broken into its components it means rua (two) and pehu (to explode or make a loud noise). But this method of breaking down Maori place names in order to explain their meaning is far from reliable.
by Bernard John Foster, M.A., Research Officer, Department of Internal Affairs, Wellington and Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.
Geology of the Tongariro Subdivision, Bulletin 40 N.S., Geological Survey Department, Gregg, D. R. (1960).