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Abstract: Recent numerical modelling studies demonstrated how pre-existing (geologically older) fault geometries within a rock volume, strongly control both the distribution of strain and fluid flow patterns during extensional fault reactivation. Fault length is particularly important with larger faults tending to accommodate more strain than smaller faults in a given population. In this paper, we explore the effects of various pore fluid pressure gradients on strain distribution and fluid flow. Our 3D models consider a simple fault architecture, with four alternative initial pore pressure gradients based on case study data from the Timor Sea. The results indicate that, in addition to geometric factors, pore fluid pressure gradients have important effects on strain localisation and fluid flow behaviour during fault reactivation. Higher pore fluid pressure gradients lead to additional strain being accommodated and increased throws on larger faults. With lower initial pore fluid pressure gradients, less strain occurs on large faults and a greater portion of the bulk strain is partitioned onto smaller faults which develop relatively larger throws. Higher pore fluid pressures can temporarily lead to greater lateral fluid migration within the reservoir and greater upward fluid discharge along large reactivated faults. Local anomalous pore fluid pressures, such as a small lateral pore pressure gradient or local overpressure within a thin layer, do not strongly impact fault reactivation results. Only high overpressures in the whole regional system seem to markedly alter strain distribution during fault reactivation. [Copyright &y& Elsevier]