The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6/fb of proton-proton ($pp$) collision data at $\sqrt{s}=7$ TeV collected by the ATLAS experiment at the Large Hadron Collider, using $t\bar{t}$ events with a $\tau$ lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching fraction ${\cal B}(H^{\pm} \rightarrow \tau\nu)$ is 100%, this leads to upper limits on ${\cal B}(t \rightarrow bH^{+})$ between 5% and 1% for charged Higgs boson masses $(m_{H^+})$ ranging from 90 to 160 GeV, respectively. In the context of the $m_h^{\text{max}}$ scenario of the MSSM, values of $\tan\beta$ larger than 13-26 are excluded for charged Higgs boson masses in the range 90~\mbox{GeV} < m_{H^+} < 150~\mbox{GeV}.

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