A large plume of steam seen throughout Rotorua last week was the result of geothermal bore testing at a site near Okere Falls.
Bay of Plenty Regional Council Compliance Manager, Sam Weiss, said work was being carried out under resource consent.
"The consent holder is currently testing a geothermal bore approximately 2km deep and this is generating the steam.
"The testing ceases when there is a westerly wind to prevent it blowing over on the road and is due to be completed this week," Weiss said.
"We understand that the consent holder has been in touch with the nearest property owners. It is a discharge of naturally occurring geothermal steam."
Bay of Plenty man Henry Phillips said a vent about 10m in diameter was "spewing" hot steam at pace when he drove past on State Highway 33, near Okere Falls, on Friday afternoon.
He said the trees on the roadside were "almost white", which he believed was due to what was coming out of the vent.
The Te Puke man was driving home from Rotorua when he saw it and said his windscreen was "absolutely covered" by the substance.
He washed it off the windscreen immediately and off the rest of the car as soon as he could out of fears it could be toxic.
GNS Science volcano information specialist Brad Scott understood this "wasn't an unprecedented hydrothermal eruption", but the product of planned geothermal activities on the land.
Rotorua Daily Post has approached the landowners for comment.