  We present results from a search for a radio transient associated with the
LIGO/Virgo source S190814bv, a likely neutron star-black hole (NSBH) merger,
with the Australian Square Kilometre Array Pathfinder. We imaged a $30\,{\rm
deg}^2$ field at $\Delta T$=2, 9 and 33 days post-merger at a frequency of
944\,MHz, comparing them to reference images from the Rapid ASKAP Continuum
Survey observed 110 days prior to the event. Each epoch of our observations
covers $89\%$ of the LIGO/Virgo localisation region. We conducted an untargeted
search for radio transients in this field, resulting in 21 candidates. For one
of these, \object[AT2019osy]{AT2019osy}, we performed multi-wavelength
follow-up and ultimately ruled out the association with S190814bv. All other
candidates are likely unrelated variables, but we cannot conclusively rule them
out. We discuss our results in the context of model predictions for radio
emission from neutron star-black hole mergers and place constrains on the
circum-merger density and inclination angle of the merger. This survey is
simultaneously the first large-scale radio follow-up of an NSBH merger, and the
most sensitive widefield radio transients search to-date.
