Geothermal fields are important to both the Australian and New Zealand economies, being responsible for most of the significant valuable ore deposits in Australia, and producing important base load electricity in New Zealand. The current state of our understanding of geothermal fields is summarised. A new regional model of geothermal fields is presented, and it is suggested that the inferred values of porosity and permeability in our regional model results mostly from the rapid cooling of intruded magma by infiltrated rainfall.