  A search for the direct production of charginos and neutralinos in final
states with at least two hadronically decaying tau leptons is presented. The
analysis uses a dataset of $pp$ collisions corresponding to an integrated
luminosity of 36.1 fb$^{-1}$, recorded with the ATLAS detector at the Large
Hadron Collider at a centre-of-mass energy of 13 TeV. No significant deviation
from the Standard Model background expectation is observed. Limits are derived
in scenarios of $\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ pair production and
of $\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{2}^{0}$ and
$\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ production in simplified models
where the neutralinos and charginos decay solely via intermediate left-handed
staus and tau sneutrinos, and the mass of the $\tilde{\tau}_{\mathrm L}$ state
is set to be halfway between the masses of the $\tilde{\chi}_{1}^{\pm}$ and the
$\tilde{\chi}_{1}^{0}$. Chargino masses up to 630 GeV are excluded at 95%
confidence level in the scenario of direct production of
$\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ for a massless
$\tilde{\chi}_{1}^{0}$. Common $\tilde{\chi}_{1}^{\pm}$, $\tilde{\chi}_{2}^{0}$
masses up to 760 GeV are excluded in the case of production of
$\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{2}^{0}$ and
$\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ assuming a massless
$\tilde{\chi}_{1}^{0}$. Exclusion limits for additional benchmark scenarios
with large and small mass-splitting between the $\tilde{\chi}_{1}^{\pm}$ and
the $\tilde{\chi}_{1}^{0}$ are also studied by varying the
$\tilde{\tau}_{\mathrm L}$ mass between the masses of the
$\tilde{\chi}_{1}^{\pm}$ and the $\tilde{\chi}_{1}^{0}$.
