  SMSS\,J114447.77-430859.3 ($z=0.83$) has been identified in the SkyMapper
Southern Survey as the most luminous quasar in the last $\sim 9\,\rm Gyr$. In
this paper, we report on the eROSITA/Spectrum-Roentgen-Gamma (SRG) observations
of the source from the eROSITA All Sky Survey, along with presenting results
from recent monitoring performed using Swift, XMM-Newton, and NuSTAR. The
source shows a clear variability by factors of $\sim 10$ and $\sim 2.7$ over
timescales of a year and of a few days, respectively. When fit with an absorbed
power law plus high-energy cutoff, the X-ray spectra reveal a $\Gamma=2.2 \pm
0.2$ and $E_{\rm cut}=23^{+26}_{-5}\,\rm keV$. Assuming Comptonisation, we
estimate a coronal optical depth and electron temperature of $\tau=2.5-5.3\,
(5.2-8)$ and $kT=8-18\, (7.5-14)\,\rm keV$, respectively, for a slab
(spherical) geometry. The broadband SED is successfully modelled by assuming
either a standard accretion disc illuminated by a central X-ray source, or a
thin disc with a slim disc emissivity profile. The former model results in a
black hole mass estimate of the order of $10^{10}\,M_\odot$, slightly higher
than prior optical estimates; meanwhile, the latter model suggests a lower
mass. Both models suggest sub-Eddington accretion when assuming a spinning
black hole, and a compact ($\sim 10\,r_{\rm g}$) X-ray corona. The measured
intrinsic column density and the Eddington ratio strongly suggest the presence
of an outflow driven by radiation pressure. This is also supported by variation
of absorption by an order of magnitude over the period of $\sim 900\,\rm days$.
