The study area is located on the eastern side of the Taupo volcanic zone in central North Island of New Zealand. It lies a few kilometres to the southwest of Mt Tarawera, the site of the biggest New Zealand volcanic eruption in historical times (the June 1886 Tarawera eruption). The study area includes the Waimangu geothermal field and a small part of Waiotapu and Waikite fields. The extensive surface thermal expressions (boiling springs, hot lakes, craters, and sinter terraces) occurring at the Waimangu field were all formed following the 1886 Tarawera eruption. Another large area of less intense thermal manifestations (thermal ground and hydrothermally altered rocks) exists about 5 km southwest of Waimangu, extending towards the Waiotapu field in the south. In 1993 an aeromagnetic survey was conducted over the study area at an average altitude of about 350 m above the ground. The results show a subdued negative residual anomaly (about -100 nT) over the Waimangu field, which can be interpreted by near-surface hydrothermal demagnetisation of rhyolitic host rocks. The lateral distribution of the demagnetised rocks is much greater than the thermal area of Waimangu, and is consistent with the extent of low resistivity rocks across the study area. The magnetic interpretation also shows that two-high standing dacite domes situated about 5 and 7 km to the southwest of Waimangu have been affected by hydrothermal demagnetisation. There are negative residual anomalies outside the low resistivity zone that could be associated with reversely magnetised rocks (age > 0.78 Ma). A strong positive residual anomaly (up to 450 nT) occurs to the east of the Waimangu field. Results from 3-D magnetic interpretation indicate some alternative models for this positive anomaly: (1) southwest-northeast trending, vertical basalt dykes (magnetisation 10 A/m), tops between -0.1 and -0.65 km RL (reduced level = relative to sea level), (2) a thick (greater than or equal to 1 km) sequence of rhyolites (magnetisation 2.5 A/m).