  We carried a detailed spectral and temporal study of blazar,
\mbox{PKS\,0903-57} using the \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT
observations, during its brightest flaring period MJD\,58931--58970. During
this period, the maximum daily averaged $\gamma$-ray flux ($\rm
F_{0.1-500\,GeV}$) of $\rm9.42\times10^{-6}\,ph\,cm^{-2}\,s^{-1}$ is observed
on MJD\,58951.5, the highest $\gamma$-ray flux detected from
\mbox{PKS\,0903-57} till now. Several high-energy (HE) photons ($>\,10$\,GeV)
consistent with the source location at high probability (>\,99\%) are detected,
and the $\gamma$-ray lightcurve in the active state shows multiple
substructures with asymmetric profile. In order to understand the possible
physical scenario responsible for the flux enhancement, we carried a detailed
broadband spectral study of \mbox{PKS\,0903-57} by choosing different flux
states from its active period. Neglecting the multi-band variability in each of
the selected time intervals, we could reproduce their averaged broadband SEDs
with a one-zone leptonic model whose parameters were derived with a
$\chi^2$-fit. We found that the broadband SED during different flux states can
be reproduced by the synchrotron, synchrotron-self-Compton (SSC) and
External-Compton (EC) processes. The seed photons for EC process from BLR or IR
torus provide acceptable fits to the GeV spectrum in all the flux states;
however, the detection of HE photons together with the equipartition condition
suggest that the EC/IR process is a more likely scenario. Further, a detailed
comparison between the fit parameters shows that the flux enhancement from
quiescent-state to the flaring-state is mostly related to increase in the bulk
Lorentz factor of the emission region and change in the break energy of the
source spectrum.
