Burns suffered by a tramper who walked off a track to get closer to a steam vent are a serious reminder to not stray off paths, the Department of Conservation says.
A senior safety ranger Theo Chapman said the tramper was badly burned last month on the Tongariro Alpine Crossing when he walked off track into a steam vent near Emerald Lakes.
The man and his partner were trying to get closer so they could take a photo.
About two metres from the top of the vent, one of the hiker's legs dropped through the surface crust, causing burns up to his knee.
He was airlifted to Waikato Hospital and received treatment for second-degree burns.
Chapman said the incident was a serious reminder for hikers to stay on the marked track.
He said the track crossed an active volcanic landscape, and there was always some risk from volcanic hazards.
"There is geothermal activity around the Tongariro Alpine Crossing with a number of steam vents present, particularly around the popular Emerald Lakes area.
"Steam vents should not be approached as they can be very hot and the ground unstable."
DOC said it had "substantial mitigation measures" in place, and visitors should make a personal choice if they were comfortable accepting the risk of hiking on an active volcano.