White Island's rising crater lake could overflow mid-way through next year, potentially posing a danger to people visiting the offshore Bay of Plenty volcano.

But scientists say that danger would be mitigated by the low likelihood of the lake's outlet failing and a flood occurring.

In a blog post, GNS Science volcanologist Brad Scott reported the water level at the lake had risen by around 10 metres this year, and would begin to overflow at some point in the middle of next year if this continued.

The island's famous sulphur-green lake, which is as acidic as battery acid, has been steadily growing since it became re-established in January.

Advertisement

The previous, much larger one was destroyed following an eruption in April 2016 and excavated the crater floor by about 13 metres.

As of this week, the lake's water level was sitting at about 10m below the point of overflow.

The lake had risen in a similar fashion three times in the last 15 years, coming close to, but never reaching overflow.

Between February and March of 2006, the lake came within about a metre of overflow, before receding as the lake heated and the water evaporated.

Scientists had earlier cautioned that the growing crater lake could trigger hydrothermal surface activity such as geysering as it drowned some of the fumaroles, or vents, near the lake.

While most of these vents seemed to have been covered quietly, Scott said the process wasn't yet over, with some vents still steaming away above lake level.

Scott said the overflow of the lake could pose a danger to people visiting the island.

The island's famous sulphur-green lake, which is as acidic as battery acid, been steadily growing since it became re-established in January. Photo / GNS Science
The island's famous sulphur-green lake, which is as acidic as battery acid, been steadily growing since it became re-established in January. Photo / GNS Science

"This danger relates to a large stream flowing across the main crater floor," he explained.

"The danger is mitigated, however by the low probability of the lake outlet failing and a flood occurring."

"Aspects of these hazards were examined in 2005 by GNS Science. We will continue to review as the lake refills."

Steep slopes and soft ground made access to the lake difficult, so scientists weren't always able to measure the water level directly and instead relied upon web camera images and other measurement techniques.

One used a topography model derived by drone imagery when the lake was not present.

"We identify the area drowned and calculate the depth. The other uses the measurement of distances below a target on the crater rim on our web camera images, from which we can calculate a water level."

Scott suspected the water level had been rising due to a combination of the condensing of volcanic steam and gas from vents under the lake, and rainfall entering the lake catchment.

"Based on our calculations and analysis of water samples, we estimate about 75 per cent of the fluid comes from condensing steam vents under the lake," he said.

"The rest comes from rainfall, which means rain has caused about 2.6 metres of the water level rise.

Plots showing the crater lake filling rates. The red dot is the current water level. Image / GeoNet
Plots showing the crater lake filling rates. The red dot is the current water level. Image / GeoNet

"Importantly, there is no substantial change in the level of volcanic activity at White Island.

"Crater Lakes have formed in the past and we do see changes in the geothermal activity associated with this, but no signs of increased volcanic unrest."

Over past decades, Scott has watched White Island transform from what it was in the mid-1970s to what it is now - a 100-metre deep, 300-metre wide crater that was dug out over a particularly fiery 12-year period now forms the sulphur-stained amphitheatre in which most activity happens today.

On any given day, the island, which rises a staggering 1.6km from the sea floor, spits out 1500 to 2000 tonnes of carbon dioxide and up to 800 tonnes of sulphur dioxide.