  The sensitivity of the ATLAS detector to the discovery of a heavy charged
Higgs boson is presented. Assuming a heavy SUSY spectrum, the most promising
channels above the top quark mass are $H^\pm\to tb$ and
$H^\pm\to\tau^\pm\nu_\tau$ which provide coverage in the low and high
$\tan\beta$ regions up to $\sim 600$ GeV. The achievable precisions on the
charged Higgs mass and $\tan\beta$ determination are also discussed. The
$H^\pm\to W^\pm h^0$ channel, though restricted to a small MSSM parameter
space, shows a viable signal in NMSSM where the parameter space is less
constrained. The observation of the channel $H^-\to\tau^-_L\nu_\tau + c.c.$ may
constitute a distinctive evidence for models with singlet neutrinos in large
extra dimensions.
