A second hydrothermal eruption in Lake Rotorua today has been captured in a video posted to the Steam and Mud Facebook page this afternoon.

The video shows surges of black mud boiling the water near the lakeshore at Ohimemutu, but is smaller than the eruption on Monday morning which was described as being higher than the Pohutu geyser at Whakarewarewa which can blow up to 30 meters.

'I'm down on the site, it's a very small event,” says GNS volcanologist Brad Scott.

He thinks it's the same location for the Monday morning eruption.'Now we know where they are happening, there's no debris on the trees nearby or the beach. I'm Pretty confident it's the same place it would be very unusual not to have them in same locations.”

Previous hydrothermal eruptions in Kuirau Park in 2000 and 2001 were both single events which were over in minutes, says Brad.

But the second eruption at Ohinemutu in days is a surprise.

'There was a significant barometric pressure drop around the time of the eruption, and that's a classic mechanism for triggering these things.

'So I'm thinking there's a reasonable chance it was triggered by the barometric pressure drop around three o'clock.”

It's possible there will be more. The location is in a pretty active and high heat flow area in the geothermal system so that possibility is always there.

'It's a relatively safe area, not an area people would normally go. It's a reasonable distance away from walking tracks, road etc.”

Hydrothermal eruptions used to take place several times a year, before the city's hydrothermal exploitation was drastically reduced.

'It's the draw-down. When pull the geothermal system down you create more steam and that leads to hydrothermal eruptions.”

The analogy he uses is boiling water in an electric jug.

If the water level in the jug is three quarters or more full, there is not much steam in the top of it. But if it is only a third full then most of the volume of the jug is steam.

'You get a lot more steam going out of the jug, that's basically how a geothermal system works.

'The main point is they are not unknown. They have happened historically. In this area they are not unusual. The part that is unusual is that it's been 15 years.”