  This paper presents a search for a new $Z^\prime$ resonance decaying into a
pair of dark quarks which hadronise into dark hadrons before promptly decaying
back as Standard Model particles. This analysis is based on proton-proton
collision data recorded at $\sqrt{s}=13$ TeV with the ATLAS detector at the
Large Hadron Collider between 2015 and 2018, corresponding to an integrated
luminosity of 139 fb$^{-1}$. After selecting events containing large-radius
jets with high track multiplicity, the invariant mass distribution of the two
highest-transverse-momentum jets is scanned to look for an excess above a
data-driven estimate of the Standard Model multijet background. No significant
excess of events is observed and the results are thus used to set 95 %
confidence-level upper limits on the production cross-section times branching
ratio of the $Z^\prime$ to dark quarks as a function of the $Z^\prime$ mass for
various dark-quark scenarios.
