The small eruption in Lake Rotorua on Saturday was no cause for concern and probably just a "plumbing issue", according to a GNS volcanologist.

A geyser appeared about 4pm on Saturday and was in the same area of Te Ruapeka Bay, off Ohinemutu, where hydrothermal eruptions occurred in late 2016 and early 2017.

GNS duty volcanologist Craig Miller said the ongoing activity at Ohinemutu was localised to that area and they were not worried in terms of any bigger implications.

"These are just natural variations that happen within the hydrothermal system that is sitting under Rotorua. From time to time you get little fluctuations in activity driven at a shallow level and it doesn't necessarily indicate any change in the whole volcano.

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"As some parts of the plumbing perhaps get clogged up it builds up a bit of pressure in another part and you get these little geysering events like this."

Miller said geysers occurred independently of anything else that was going on and it was nothing of concern.

Changes in lake levels could also have an effect, he said, as it added a "pressure load" on top of the system.

"It's like squashing something down with your foot. When you take your foot off it springs back a little bit. Hydrothermal systems are often quite sensitive to that sort of thing."

Miller said the public should use common sense when it came to being in the area.

"These things happen. It's a geothermal area and it's exactly the kind of thing you expect now and again. You probably don't want to go wading out in the lake in that area because there could be boiling water."

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Miller encouraged members of the public to contact GNS if they saw any unusual geothermal activity.