GNS Science says the large plume of steam seen as far as 20km away from Rotorua last week was planned geothermal activity near Okere Falls.
Bay of Plenty resident Henry Phillips drove past the plume and told Newshub the substance coming out was falling like "light rain" and landing on his car.
"The stuff was landing on our windscreen and obviously all over the car, we had to put the wipers on to clear the windscreen."
He said plants and trees surrounding the large plume of steam were covered in "white stuff".
He was worried the white deposits falling on his car would erode its parts, so gave it a wash when he got home.
Phillips added the vent that the steam was coming out of was about 10m in diameter.
"My wife was with me, and when we saw it we thought 'crikey, look at that!'"
Volcano information specialist at GNS Science Brad Scott told Newshub he understood that it wasn't a "hydrothermal eruption".
Instead the substance spewing out was "white deposits" that are "typical by-products of bore testing".
Scott added the plume is likely the result of planned geothermal testing on privately-owned land.