That is less than a quarter of the acceptable standard.
Thirty-three per cent or less is considered earthquake-prone, 34 to 67 per cent is considered an earthquake risk and 68 per cent or more is considered acceptable.
The council revealed the assessment and tests had cost $84,618 to date.
In the statement, the council said it had only just announced the closure because "council has been in discussions with the leaseholder about the ground conditions under the building since 2019. A decision was made to close the building in January and we remain in discussions with the leaseholder about the implications of this."
Report findings, coupled with earthquakes experienced in Rotorua on January 25, led to the decision to close the building to ensure people's safety.
The council then commissioned an urgent detailed seismic assessment of the building, as well as additional ground testing, the written statement said.
Results from the assessments received by the council in April confirmed the building was earthquake-prone at 15 per cent of building standards.
The ground under the Blue Baths could be liquefiable in a large earthquake event.
Rotorua Lakes Council's operations group manager and deputy chief executive, community wellbeing Jocelyn Mikaere said while the assessment was disappointing, the safety of people needed to come first.
"We know the Blue Baths is an important building to the Rotorua community but with the building being assessed as earthquake-prone, we have made the decision to keep its doors closed for now.
"We deem the risk to allow a business to continue to operate from the building, in its current state, as unacceptable.
"We will now be working through our options to determine the future of the Blue Baths."
Fencing has been placed around the building to ensure people's safety but there is no immediate risk to people standing near the building.
