The central North Island is known for its hydrothermal activity, but GNS Science is calling the latest movements on Lake Rotorua "unusual".
Deep rumbling and geysers of up to 30 metres saw some Ohinemutu locals head for the hills yesterday, believing the lake was about to blow.
Volcanologist Brad Scott says the activity is unexpected as there hasn't been anything for 15 years.
"Hydrothermal eruptions are pretty significant. They're quite violent, they're quite hot. This one was fortuitously was offshore - it wasn't in the built-up area. They can be damaging and they can be dangerous."
The Rotorua Lakes Council said the geyser was "spectacular but not a concern". Mr Scott says it's almost impossible to say whether the activity is a precursor for a bigger explosion.