  We present high cadence detections of two superflares from a bright G8 star
(V = 11.56) with the Next Generation Transit Survey (NGTS). We improve upon
previous superflare detections by resolving the flare rise and peak, allowing
us to fit a solar flare inspired model without the need for arbitrary break
points between rise and decay. Our data also enables us to identify
substructure in the flares. From changing starspot modulation in the NGTS data
we detect a stellar rotation period of 59 hours, along with evidence for
differential rotation. We combine this rotation period with the observed
\textit{ROSAT} X-ray flux to determine that the star's X-ray activity is
saturated. We calculate the flare bolometric energies as
$5.4^{+0.8}_{-0.7}\times10^{34}$ and $2.6^{+0.4}_{-0.3}\times10^{34}$ erg and
compare our detections with G star superflares detected in the \textit{Kepler}
survey. We find our main flare to be one of the largest amplitude superflares
detected from a bright G star. With energies more than 100 times greater than
the Carrington event, our flare detections demonstrate the role that
ground-based instruments such as NGTS can have in assessing the habitability of
Earth-like exoplanets, particularly in the era of \textit{PLATO}.
