  We consider a 5D extension of the DFSZ axion model that addresses both the
axion quality and fermion mass hierarchy problems, and predicts
flavour-dependent, off-diagonal axion-fermion couplings. The axion is part of a
5D complex scalar field charged under a U(1)$_{PQ}$ symmetry that is
spontaneously broken in the bulk, and is insensitive to explicit PQ breaking on
the UV boundary. Bulk Standard Model fermions interact with two Higgs doublets
that can be localized on the UV boundary or propagate in the bulk to explain
the fermion masses and mixings. When the Higgs doublets are localized on the UV
boundary, they induce flavour diagonal couplings between the fermions and the
axion. However, when the Higgs doublets propagate in the bulk, the overlap of
the axion and fermion profiles generates flavour off-diagonal couplings. The
effective scale of these off-diagonal couplings in both the quark and lepton
sectors can be as small as $10^{11}$ GeV, and therefore will be probed in
future precision flavour experiments.
