  We analyze the hadronization structure of $\tau \to \eta \pi^- \pi^0
\nu_\tau$ decays. In the isospin limit only the vector current contributes to
this process. We compute the relevant form factor within Resonance Chiral
Theory, at leading order in the $1/N_C$ expansion, and considering only the
contribution of the lightest vector resonances. The couplings in the resonance
theory are constrained by imposing the asymptotic behaviour of vector spectral
functions ruled by $QCD$. We reproduce the branching ratio of this mode and
predict the low-energy behaviour of
$\sigma\left(e^+e^-\to\eta\pi^+\pi^-\right)$ using isospin symmetry.
