The results of a search for charged Higgs bosons decaying to a tau lepton and a neutrino, $H^{\pm} \to \tau^{\pm} \nu$, are presented. The analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark. The final state is characterised by the presence of a hadronic $\tau$ decay, missing transverse momentum, $b$-jets, a hadronically decaying $W$ boson, and the absence of any electrons or muons. The data are consistent with the expected background from Standard Model processes. This leads to 95 % confidence level upper limits on the branching ratio of top-quark decays to a $b$ quark and a charged Higgs boson times branching ratio, ${\cal B}(t\rightarrow bH^\pm) \times {\cal B}(H^\pm\rightarrow \tau^{\pm}\nu)$, between 0.23 % and 1.3 % for charged Higgs boson masses in the range $80-160$ GeV. This also leads to 95 % confidence level upper limits on the production cross section times branching ratio, $\sigma(pp \to tH^{\pm} + X) \times {\cal B}(H^{\pm} \to \tau^{\pm} \nu)$, between 0.0045 pb and 0.76 pb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV.

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