  We use the second data releases of the ESA Gaia astrometric survey and the
high-resolution GALAH spectroscopic survey to analyse the structure of our
Galaxy's disc components. With GALAH, we separate the alpha-rich and alpha-poor
discs (with respect to Fe), which are superposed in both position and velocity
space, and examine their distributions in action space. We study the
distribution of stars in the zV_z phase plane, for both V_phi and V_R, and
recover the remarkable "phase spiral" discovered by Gaia. We identify the
anticipated quadrupole signature in zV_z of a tilted velocity ellipsoid for
stars above and below the Galactic plane. By connecting our work with earlier
studies, we show that the phase spiral is likely to extend well beyond the
narrow solar neighbourhood cylinder in which it was found. The phase spiral is
a signature of corrugated waves that propagate through the disc, and the
associated non-equilibrium phase mixing. The radially asymmetric distribution
of stars involved in the phase spiral reveals that the corrugation, which is
mostly confined to the alpha-poor disc, grows in z-amplitude with increasing
radius. We present new simulations of tidal disturbance of the Galactic disc by
the Sagittarius (Sgr) dwarf. The effect on the zV_z phase plane lasts >2 Gyr
but a subsequent disc crossing wipes out the coherent structure. We find that
the phase spiral was excited < 0.5 Gyr ago by an object like Sgr with total
mass 3 x 10^10 Msun (stripped down from 5 x 10^10 Msun when it first entered
the halo) passing through the plane.
