  Optical, near-infrared (NIR) photometric and spectroscopic studies, along
with the optical imaging polarimetric results for SN 2012au, are presented in
this article to constrain the nature of the progenitor and other properties.
Well-calibrated multiband optical photometric data (from $-$0.2 to +413 d since
$B$-band maximum) were used to compute the bolometric light curve and to
perform semi-analytical light-curve modelling using the $\texttt{MINIM}$ code.
A spin-down millisecond magnetar-powered model explains the observed
photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric
follow-up observations ($-$2 to +31 d) and comparison with other similar cases
indicate signatures of asphericity in the ejecta. Good spectral coverage of SN
2012au (from $-$5 to +391 d) allows us to trace the evolution of layers of SN
ejecta in detail. SN 2012au exhibits higher line velocities in comparison with
other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf$-$Rayet
star as the possible progenitor for SN 2012au, with oxygen, He-core, and
main-sequence masses of $\sim$1.62 $\pm$ 0.15 M$_\odot$, $\sim$4$-$8 M$_\odot$,
and $\sim$17$-$25 M$_\odot$, respectively. There is a clear absence of a first
overtone of carbon monoxide (CO) features up to +319 d in the $K$-band region
of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most
luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass
($\sim$4.7$-$8.3 M$_\odot$) and kinetic energy ($\sim$[4.8 $-$ 5.4] $\times$
10$^{51}$ erg). Detailed modelling using $\texttt{MESA}$ and the results
obtained through $\texttt{STELLA}$ and $\texttt{SNEC}$ explosions also strongly
support spin-down of a magnetar with mass of around 20 M$_\odot$ and
metallicity Z = 0.04 as a possible powering source of SN 2012au.
