The 1995-96 eruptions of Ruapehu ejected a total of 60 million cubic meters of acidic ash, blanketing districts up to 300km from the mountain. At times the ash plume reached as high at 10km, which represented a significant aviation hazard.

Volcanic explosions that wrecked havoc on the Central North Island in the mid-1990s will be remembered this weekend in Tongariro.

Scientists, emergency managers, conservation staff, iwi, and university students will gather at Whakapapa Village to mark 20-years since Mt Ruapehu erupted in 1995 and 1996.

This year is also the 40th anniversary of the 1975 Ngauruhoe eruption and the 70th anniversary of the 1945 Ruapehu eruption.

The purpose of the commemoration is to reflect on what we've learned from the past, said volcano information specialist at GNS Science, Brad Scott.

The Ruapehu eruptions ejected a reported 60 million cubic meters of acidic ash, which wrecked havoc on districts up to 300 kilometres from the mountain.

At times the ash plume reached as high at 10km.

Scott said the 1995-96 eruptions were similar in size to those in 1945, but their social and economic impacts were much greater.

The ash irritated eyes and throats, damaged car paintwork and machinery, contaminated rivers and water supplies, ruined crops, closed state highways, forced airports to shut, and killed livestock which ate ash-covered pastures.

"In the 1940s we didn't have multiple aviation flights, or freight trucks on State Highway 1. We didn't have that fast consumer goods society where supermarkets get stocked at night," Scott said.

Fortunately the major eruptive activity occurred while the ski areas were closed, so there were no deaths or serious injuries.

"Some of the big lessons that came out of it were managing airspace," Scott said.

Starting on August 14, the Volcanic Science and Risk Management Workshop will feature a range of science presentations and activities to reflect on two decades of progress in managing eruption hazards and risks in Tongariro National Park.

The main workshop is scheduled for August 14, while other workshops will continue over the weekend at Tongariro National Park Visitor Centre, Whakapapa Village.

As part of the programme, a public talk is scheduled for August 15 at 6.30pm at Lorenz's Bar and Cafe at the top of the Bruce Road, Whakapapa.

"We believe this is a great opportunity to reflect on two decades of progress in monitoring, scientific understanding, risk management and social science," said director of the Joint Centre for Disaster Research, Dr David Johnston.

Scott said Tongariro National Park is a highly active volcanic centre and, while scientists aren't expecting another eruption "within hours to weeks," it would be unwise to think of the park as a benign playground.

Scott said the central North Island volcanoes are now better monitored than ever before with seismographs, GPS and web cameras placed throughout the National Park, keeping watch on the volcanic pulse of the area.

GeoNet's volcano teams also regularly visit active areas such as Ruapehu's Crater Lake, Ngauruhoe's crater and the Te Maari craters to collect water samples for analysis and to check Temperatures.

This is important for tracking long-term trends and picking up early signs of unusual behaviour or volcanic unrest.

The steady flow of monitoring data enables the Department of Conservation and other agencies to respond quickly to manage the risks.