  We present a Monte Carlo radiative transfer technique for calculating
synthetic spectropolarimetry for multi-dimensional supernova explosion models.
The approach utilises "virtual-packets" that are generated during the
propagation of the Monte Carlo quanta and used to compute synthetic observables
for specific observer orientations. Compared to extracting synthetic
observables by direct binning of emergent Monte Carlo quanta, this
virtual-packet approach leads to a substantial reduction in the Monte Carlo
noise. This is vital for calculating synthetic spectropolarimetry (since the
degree of polarisation is typically very small) but also useful for
calculations of light curves and spectra. We first validate our approach via
application of an idealised test code to simple geometries. We then describe
its implementation in the Monte Carlo radiative transfer code ARTIS and present
test calculations for simple models for Type Ia supernovae. Specifically, we
use the well-known one-dimensional W7 model to verify that our scheme can
accurately recover zero polarisation from a spherical model, and to demonstrate
the reduction in Monte Carlo noise compared to a simple packet-binning
approach. To investigate the impact of aspherical ejecta on the polarisation
spectra, we then use ARTIS to calculate synthetic observables for prolate and
oblate ellipsoidal models with Type Ia supernova compositions.
