Distinct magnetic total force anomalies are associated with many, but not all, liquid-dominated geothermal reservoirs in the Taupo Volcanic Zone, which stand in thick, young (<0.7 Ma) volcanic host rocks. The anomalies are caused by demagnetised rocks (a result of hydrothermal alteration) in the upper 0.5-1 km section of the reservoirs; the phenomenon has been confirmed by core studies. Four different patterns of magnetic anomalies have been recognised. The most common type can be found over high-temperature systems where demagnetisation is confined to the main reservoir. Different anomaly patterns are associated with prospects characterised by subsurface outflows and reservoirs hosted in both normally and reversely magnetised rocks. A rather subdued and almost featureless pattern occurs over a few prospects where rocks lying outside the reservoir have also lost their magnetisation by interaction with acidic, steam-heated water.