  We present the results of an Australia Telescope Compact Array 1.4 GHz
spectropolarimetric aperture synthesis survey of 34 square degrees centred on
Centaurus A - NGC 5128. A catalogue of 1005 extragalactic compact radio sources
in the field to a continuum flux density of 3 mJy/beam is provided along with a
table of Faraday rotation measures (RMs) and linear polarised intensities for
the 28 percent of sources with high signal-to-noise in linear polarisation. We
use the ensemble of 281 background polarised sources as line-of-sight probes of
the structure of the giant radio lobes of Centaurus A. This is the first time
such a method has been applied to radio galaxy lobes and we explain how it
differs from the conventional methods that are often complicated by depth and
beam depolarisation effects. We use an RM structure function analysis and
report the detection of a turbulent RM signal, with rms of 17 rad/m^2 and scale
size 0.3 degrees, associated with the southern giant lobe. We cannot verify
whether this signal arises from turbulent structure throughout the lobe or only
in a thin skin (or sheath) around the edge, although we favour the latter. The
RM signal is modelled as possibly arising from a thin skin with a thermal
plasma density equivalent to the Centaurus intragroup medium density and a
coherent magnetic field that reverses its sign on a spatial scale of 20 kpc.
