Geophysical data show that the southern part of the Hauraki Depression is a rift structure. In W—E cross section it is made up of a fault angle depression, a median horst, and a graben. A maximum thickness of about 3 km of Quaternary and Tertiary sediments fill the two depressions. The fault angle depression and the graben are bounded at the eastern side by major normal faults which dip about 70 ± 10° to the west, namely, the recently discovered Kerepehi Fault in the centre and the Hauraki Fault which forms the eastern boundary of the depression. A minor hinge fault (Firth of Thames Fault) probably runs along the western boundary. Transverse faults cross the depression causing horizontal offsets of the major normal faults of up to 3 km. Recent rifting of the southernmost part is indicated by active faulting and by shallow earthquake activity beneath the depression (focal depth ≤ 12 km). The chemistry of numerous hot springs implies that high temperatures (250–300°c) occur at depths of less than 5 km; these springs occur over the boundary faults and over the median basement ridge.