A fault in a nearby bore caused the eruption of mud, steam and water on Lake Rd on Wednesday, Rotorua Lakes Council has confirmed.
A volcanologist has described the emergence of the fumarole - a steam-driven vent - as apparent "infrastructure failure", while a council expert said the event was "man-made".
The geothermal activity in the median-strip garden on Lake Rd near Kuirau Park has now stopped, but there may still be steam in the area as the ground cools.
Confirmation of the cause of the fumarole came after the deep quench of a nearby bore, which caused water levels in the fumarole to drop significantly, the council said in a statement.
Well drillers were being organised to grout the bore shut, which was expected to happen tomorrow, the council said.
The council said the section of Lake Rd will remain closed and traffic will keep being diverted down Tarewa Rd.
The council's geothermal inspector Peter Brownbridge earlier told Rotorua Lakes councillors in an Operations and Monitoring Committee meeting today that the incident was "a man-made event".
He said there was a "geothermal breakdown".
He said the immediate area was not known for hot ground, fumaroles or springs and said one of his first actions on Wednesday was cranking three nearby geothermal wells, looking for an effect.
"We have pinpointed one well in particular which we think may have a broken casing at depth."
He said one of the reasons the team looked hard at the wells was because the water was 98C at the surface and kicking up the clays that came from "great depth", which indicated it was "more than just the naturally-fed fumarole".
He said rods were being run down the suspect well and would do a deep quench, which involved injecting large volumes of cold water down it to cool the geothermal activity.
This would prove whether or not this bore was responsible for the fumarole.
If this was the case, once the bore is quenched, the geothermal thermal activity within the fumarole should stop, allowing it to be filled and the road to be reopened.
Once the bore was quenched, they could better understand what type of fault caused the fumarole. A break deep in the bore casing was suspected.
He said there was no overflow and the roads have been cleaned, but the two lanes coming into town remained shut.
He said a lot of time was spent stopping any sediments and dirty water from getting into the Utuhina Stream. Three sucker trucks and a 10-tonne truck removed sediment from the site.
Councillor Tania Tapsell was concerned that the area was not known for fumaroles or producing much heat, and asked if this could signal potential geothermal activity in areas that were thought to not be active.
Brownbridge was "not concerned because this isn't a natural feature. This is, if you like, a man-made event," he said.
"The well that we're looking at has 89m of casing and just over 100m of open hole underneath it."
Councillor David Donaldson asked if there needed to be testing under the road before it was reopened, unsure if the event would have caused any erosion.
Brownbridge said they carefully monitored any movement of the ground, particularly when heavy traffic drove past and had not encountered any signs of irregular movement.
Councillor Trevor Maxwell asked if it was a "water table issue" and Brownbridge responded that the feature was coming from depth and the surface water was not the effect or cause.
Yesterday, Brownbridge said there had not been any previous reports of hot springs in the area.
GNS volcanologist Brad Scott said on Thursday the incident "appears to be an infrastructure failure of a local geothermal bore".
He said GNS had not made a detailed inspection but it did not appear to be a natural event, with around 80-plus per cent of these types of events not being natural activities.
If it was an infrastructure failure, he said this could not be predicted.
Bay of Plenty Regional Council said the event resulted in mud and geothermal fluid flowing into the Utuhina Stream.
"[This] may mean there is some visible discolouration. We'll be keeping an eye on it, taking water quality samples and monitoring the temperature, to identify any impacts on the awa [stream]."