The seismic activity in the Koyna area is clearly related to the water impoundment of the Koyna dam in 1962, and has reached a remarkable level with the occurrence of a major event of magnitude 6.3 in 1967 December 10. We present a homogeneous poroelastic model based on analysis of the first eight years of seismicity, which aims to link the water-level fluctuations of the reservoir with the seismicity. Starting from a discretized lake,we calculate the stress field resulting from the water-level fluctuations and the pore pressure changes due to the undrained and the diffusive responses of the medium. Then, we compare the Coulomb stress variations with a set of relocated seismic events. We find that more than 80 per cent of the relocated events before the M6.3 event are well described by this poroelastic model, leading us to derive a suitable diffusivity cp = 0.2m2 s-1. Then, we model the response of the system after the M6.3 event of 1967 December 10, by comparing the variation of the Coulomb stress field with the spatio-temporal characteristics of the relocated post-seismic events and the decay of aftershocks with time. We find that compared to before the main event a tenfold increase in hydraulic diffusivity is required to satisfactorily describe the aftershock decay with the appropriate Omori exponent. Although this increase in diffusivity may be physically related to the main shock we also note that events later than 9 months after the main shock are not well explained. We therefore propose an alternative hydrological model, which involves two compartments of contrasting diffusivities.