  We define and test $\mathrm{CP}$-even and $\mathrm{CP}$-odd partial
differential widths for the process $\tau\rightarrow K\pi\pi\nu_{\tau}$
assuming that an intermediate heavy charged scalar contributes to the decay
amplitude. Adopting a model-independent approach, we use a Monte Carlo
simulation in order to study the number of events needed to recover information
on the new physics from these observables. Our analysis of the
$\mathrm{CP}$-odd observables indicates that the magnitude of $f_H \eta_P$,
which is related to the new-physics contribution, can be recovered with an
uncertainty smaller than $3%$ for $3\times 10^6$ events. This number of events
would also allow one to retrieve certain parameters appearing in the SM
amplitude at the percent level. In addition, we discuss the possibility of
using the proposed observables to study specific models involving two Higgs
doublets, such as the aligned two-Higgs-doublet model (A2HDM). This analysis is
undertaken within the context of the upcoming Super B-factories, which are
expected to provide a considerably larger number of events than that which was
supplied by the B-factories. Moreover, a similar set of observables could be
employed to study other decay modes such as
$\tau\rightarrow\pi\pi\pi\nu_{\tau},\,\tau\rightarrow KK\pi\nu_{\tau}$ and
$\tau\rightarrow KKK\nu_{\tau}$.
