  We identify a counterpart to a Galactic supershell in diffuse radio
polarisation, and use this to determine the magnetic fields associated with
this object. GSH 006$-$15$+$7 has perturbed the polarised emission at
2.3$\,$GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS),
acting as a Faraday screen. We model the Faraday rotation over the shell, and
produce a map of Faraday depth over the area across it. Such models require
information about the polarised emission behind the screen, which we obtain
from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23$\,$GHz to
2.3$\,$GHz, to estimate the synchrotron background behind GSH 006$-$15$+$7.
Using the modelled Faraday thickness we determine the magnitude and the
plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We
find a peak line-of-sight field strength of $|B_\parallel|_\text{peak} =
2.0\substack{+0.01 \\ -0.7}\,\mu$G. Our measurement probes weak magnetic fields
in a low-density regime (number densities of $\sim0.4\,$cm$^{-3}$) of the ISM,
thus providing crucial information about the magnetic fields in the
partially-ionised phase.
