  We study the metastability and decay of multiply-charged superflow in a
ring-shaped atomic Bose-Einstein condensate. Supercurrent corresponding to a
giant vortex with topological charge up to q=10 is phase-imprinted optically
and detected both interferometrically and kinematically. We observe q=3
superflow persisting for up to a minute and clearly resolve a cascade of
quantised steps in its decay. These stochastic decay events, associated with
vortex-induced $2 \pi$ phase slips, correspond to collective jumps of atoms
between discrete q values. We demonstrate the ability to detect quantised
rotational states with > 99 % fidelity, which allows a detailed quantitative
study of time-resolved phase-slip dynamics. We find that the supercurrent
decays rapidly if the superflow speed exceeds a critical velocity in good
agreement with numerical simulations, and we also observe rare stochastic phase
slips for superflow speeds below the critical velocity.
