The Ministry began environmental radioactivity monitoring in the New Zealand and South Pacific regions to assess radioactive fallout from nuclear weapons tests.
By 1985, levels of weapons-test debris had decreased to near the limits of detection for the monitoring techniques in use at the time. Monitoring was scaled down to three stations at Kaitaia, Hokitika and Rarotonga. More sensitive sampling equipment was installed.
New Zealand signed the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in 1996 and enacted the Nuclear-Test-Ban Act in 1999. To enforce the CTBT, seismic, radionuclide, infrasound and hydro-acoustic monitoring stations were set up around the world.
In 2000, the Kaitaia and Rarotonga stations were upgraded, and a new station was commissioned at Chatham Island. These stations now provide continuous daily monitoring with high-sensitivity air samplers.
The Ministry also monitored rainwater in Hokitika and milk powder in Waikato, Taranaki and Westland for evidence of radioactivity.
In 2004 the Ministry stopped monitoring levels of strontium-90 in milk powders because the concentrations had fallen below detectable levels.
In 2012/13, GNS Science provided information about hydrogen-3 and carbon-14 levels collected from its stations at Baring Head and Kaitoke, in Wellington.
Marine monitoring commenced in 2015 with seawater samples analysed for the presence of Cs-134 and Cs-137.