The Whakatane Graben is a back-arc basin situated in the Bay of Plenty, North Island, New Zealand, associated with the oblique westward subduction of the Pacific Plate beneath the Australian Plate. The data presented in this thesis reveal both long-term and short-term normal fault behaviour in a fault population surrounding and including one of the most active faults in the offshore graben. Closely spaced (100 - 200 m separation) multichannel seismic and high resolution (chirp sonar, 3.5 kHz and boomer) profiles were collected during cruises in December 1999 and January 2001. These data were processed and interpreted and fault displacement data were extracted. The Rangitaiki Fault lies in the centre of the Whakatane Graben, and is now recognised as a 20 km long, linked, segmented normal fault. The displacement profile of the fully linked Rangitaiki Fault resembles that of a single fault, with a maximum close to the centre and displacements decreasing towards the tips. The fault was found to have initiated as 5 isolated segments, which grew together over 1 Ma. Fault tip propagation was the dominant faulting process initially, followed by the development of relay zones and their subsequent breaching. The system became fully linked between 300 ka and 17 ka, and increased in displacement rate from 0.52 ± 0.18 mm yr⁻¹ prior to linkage, to 1.41 ± 0.31 mm yr⁻¹ afterward. Accurate observations of fault growth rates are hampered by the limited age control of the seismic horizons. The fault propagated northwards and the fault system present today is interpreted as a highly evolved damage zone, where optimally oriented and located faults have linked to form the major Rangitaiki Fault structure and have increased in displacement rate. Displacement is transferred southward from the Rangitaiki Fault onto the newly identified Thornton Fault. The Thornton Fault is along-strike from the onshore surface ruptures caused by the 1987 Edgecumbe earthquake (magnitude 6.3). A repeat time of 320-600 years is inferred for Edgecumbe size events on the Rangitaiki Fault, close to the maximum magnitude for that size of fault. The combined extension rate for all observed faults across the pseudo-3D survey area for the last 17 ± 1 ka is 2.4-3.4 mm yr⁻¹, increased from 0.7-0.9 mm yr⁻¹ for the time interval between 300-17 ka. The increase in extension rate observed in the Whakatane Graben is interpreted as resulting from the eastward migration of the locus of deformation within the Bay of Plenty towards the Hikurangi subduction zone.