We present the evolution in the number density and stellar mass functions of photometrically selected post-starburst galaxies in the UKIDSSUltraDeep Survey,with redshifts of 0.510. We find that this transitionary species of galaxy is rare at all redshifts, contributing ∼5 per cent of the total population at z ∼ 2, to /~ 2 they are exclusively massive galaxies that have formed the bulk of their stars during a rapid assembly period, followed by complete quenching of further star formation; (2) at z ~1 they are caused by the rapid quenching of gas-rich star-forming galaxies, independent of stellar mass, possibly due to environment and/or gas-rich major mergers.