Ohinemutu is always blanketed in steam, but a fresh eruption in the lake this weekend has frightened a few residents.

Saturday afternoon, about 4pm, Maaka Maxwell, 10, and his brother Ari, 11, came running into the house yelling that the lake had erupted.

"There was like a geyser in the lake," Ari said.

"Smoke was coming out of it but it wasn't really scary."

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The boys had just gone outside with their cousins when they saw it.

"I've not really seen anything like that before," Maaka said.

The geyser came up in Te Ruapeka Bay.

In April last year, a new ngāwha (geyser of boiling water) appeared in front of St Faith's church.

It followed a series of hydrothermal eruptions in Lake Rotorua in late 2016 and early 2017.

The eruptions were near Ohinemutu Village and were described as geysers of water emerging from the lake.

Maaka and Ari's mum Rongopai Stirling-Maxwell said with the previous eruptions there had been an "explosion noise" but there was nothing like that yesterday.

"I thought they were just fibbing, but there were a few people that saw it and it was almost straight away after they'd gone out.

"It's a bit scary because no one has really made it clear why it's happening.

"We assume it isn't too dangerous because we've not been told to evacuate."

A GNS scientist could not be reached for comment.