Beneath Hawke's Bay, the interface between the subducted Pacific and overlying Australian plates lies at shallow depth - within the depth range where large subduction thrust earthquakes are expected. Determining the likely size of such earthquakes is thus a major issue in quantifying the seismic hazard of the region. Here we use recent seismological, geodetic and geologic research results to estimate the rupture dimensions, magnitude and recurrence of a large subduction thrust event. The estimated rupture zone extends 45 km downdip, from 15 km to 22 km depth on the plate interface, and 120 km along strike, from 45 km southwest of Napier to 10 km northeast of Wairoa. This equates to an Mw 7.7 earthquake. The estimated recurrence of such an event depends on the coupling coefficient (i.e. the ratio of seismic slip to total slip) at the plate interface, which is not well determined. Our preferred range for this coefficient is 0.3-0.5, which yields a recurrence interval range of 250-400 years. Such recurrence is broadly consistent with the geological record of subsidence in the Ahuriri Lagoon near Napier in the 3500 years prior to the Ms 7.8 Hawke's Bay earthquake of 1931.