  We used the Australia Telescope Compact Array to map a large field of
approximately $2^{\circ} \times 2^{\circ}$ around the Sculptor group galaxy
NGC~300 in the 21-cm line emission of neutral hydrogen. We achieved a $5
\sigma$ \ion{H}{i} column density sensitivity of $10^{19}~\mathrm{cm}^{-2}$
over a spectral channel width of $8~\mathrm{km \, s}^{-1}$ for emission filling
the $180'' \times 88''$ synthesised beam. The corresponding \ion{H}{i} mass
sensitivity is $1.2 \times 10^{5}~\mathrm{M}_{\odot}$, assuming a distance of
$1.9~\mathrm{Mpc}$. For the first time, the vast \ion{H}{i} disc of NGC~300 has
been mapped over its entire extent at a moderately high spatial resolution of
about $1~\mathrm{kpc}$.
  NGC~300 is characterised by a dense inner \ion{H}{i} disc, well aligned with
the optical disc of $290^{\circ}$ orientation angle, and an extended outer
\ion{H}{i} disc with a major axis of more than $1^{\circ}$ on the sky
(equivalent to a diameter of about $35~\mathrm{kpc}$) and a different
orientation angle of $332^{\circ}$. A significant fraction (about 43~per cent)
of the total detected \ion{H}{i} mass of $1.5 \times 10^{9}~\mathrm{M}_{\odot}$
resides within the extended outer disc. We fitted a tilted ring model to the
velocity field of NGC~300 to derive the rotation curve out to a radius of
$18.4~\mathrm{kpc}$, almost twice the range of previous rotation curve studies.
The rotation curve rises to a maximum velocity of almost $100~\mathrm{km \,
s}^{-1}$ and then gently decreases again in the outer disc beyond a radius of
about $10~\mathrm{kpc}$. Mass models fitted to the derived rotation curve yield
good fits for Burkert and NFW dark matter halo models, whereas
pseudo-isothermal halo models and MOND-based models both struggle to cope with
the declining rotation curve.
  We also observe significant asymmetries in the outer \ion{H}{i} disc of
NGC~300, in particular near the edge of the disc, which are possibly due to ram
pressure stripping of gas by the intergalactic medium (IGM) of the Sculptor
group. Our estimates show that ram pressure stripping can occur under
reasonable assumptions on the density of the IGM and the relative velocity of
NGC~300. The asymmetries in the gas disc suggest a proper motion of NGC~300
toward the south-east. At the same time, our data exclude IGM densities of
significantly higher than $10^{-5}~\mathrm{cm}^{-3}$ in the vicinity of
NGC~300, as otherwise the outer gas disc would have been stripped.
