  Fast Radio Bursts (FRBs) are extragalactic transients of (sub-)millisecond
duration that show wide-ranging spectral, temporal, and polarimetric
properties. The polarimetric analysis of FRBs can be used to probe intervening
media, study the emission mechanism, and test possible progenitor models. In
particular, low frequency depolarisation of FRBs can identify dense, turbulent,
magnetised, ionised plasma thought to be near the FRB progenitor. An ensemble
of repeating FRBs has shown low-frequency depolarisation. The depolarisation is
quantified by the parameter $\sigma_{\rm RM}$, which correlates with proxies
for both the turbulence and mean magnetic field strength of the putative
plasma. However, while many non-repeating FRBs show comparable scattering (and
hence inferred turbulence) to repeating FRBs, it is unclear whether their
surrounding environments are comparable to those of repeating FRBs. To test
this, we analyse the spectro-polarimetric properties of five one-off FRBs and
one repeating FRB, detected and localised by the Australian Square Kilometer
Array Pathfinder. We search for evidence of depolarisation due to $\sigma_{\rm
RM}$ and consider models where the depolarisation is intrinsic to the source.
We find no evidence (for or against) the sample showing spectral
depolarisation. Under the assumption that FRBs have multipath
propagation-induced depolarisation, the correlation between our constraint on
$\sigma_{\rm RM}$ and RM is consistent with repeating FRBs only if the values
of $\sigma_{\rm RM}$ are much smaller than our upper limits. The observations
provide further evidence for differences in the environments and sources of
one-off and repeating FRBs.
