This week marks 101 years since 11 sulphur miners were killed by a lahar on White Island in the Bay of Plenty. Legend has it that the only survivor of the 1914 disaster was a miners' cat named Peter the Great.

The sulphur was processed at a works in Tauranga at Sulphur Point, near Sanford Ltd. Today, White Island remains New Zealand's largest and most active volcano.

The ruins of the White Island sulphur works, destroyed in 1914. Photos: Tracy Hardy.

Because of continuing activity, Geonet's gas team make monthly flights over White Island, Ruapehu, Ngauruhoe and Tongariro, to measure the gases being produced.

Since the last minor eruptions, from August-October 2013, the amount of gas has declined slightly, says Geonet volcanologist Brad Scott.

'Since the eruptions, the carbon dioxide output has ranged from 950-to-1800 tons per day,” explains Brad, 'while sulphur dioxide ranges from 170-to-610 tons per day and hydrogen sulphide ranges from 4-to-18 tons per day.

'On Friday we measured a daily gas flux of 806 tons of CO2, 217 tons of SO2 and 10.4 tons of H2S.”

Geonet also has two automated SO2 gas sensors at White Island and obtain two-to-three days' worth of data each week from them. The SO2 output measured by them has ranged from 106-to-675 tons per day, with last week's data typical of the environment.

'White island is by far the easiest volcano for us to measure as it always has some gas coming out of it and there are no other hills around,” adds Brad.

Instruments are mounted on either a Cessna 206 or a Scenica aircraft, and while some of the instruments ‘sniff' the gas, others look up through the volcanic gas plume.

Video footage of the team in action is available, with some spectacular aerial views of the volcanoes as the aircraft's instruments measures the volcanic CO2, SO2 and H2S.

The now uninhabited White Island is around 2 km in diameter and 48 km from the coast of the Bay of Plenty. It marks the northern end of the Taupo Volcanic Zone.

Above sea level, White Island is 321m high, but stands in water about one kilometre deep. Most of the island is occupied by the main crater, with the crater floor being less than 30m above sea level.

White Island has been active for at least 150,000 years, and is a composite cone stratovolcano made of layers of andesite lava flows and pyroclastic ash deposits. Since human settlement in New Zealand, there has been continual low-level activity and small eruptions.

From 1975-to-2001, there were frequent small eruptions, making this the island's most active period in hundreds of years.

Ash and gas plumes rose as high as 10km, while lava bombs and blocks were thrown into the sea and the glow of red hot rock was visible at night from the Bay of Plenty coastline.