This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying $\tau $-lepton pair with low transverse momentum. When two $\tau $-leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the $\tau $-leptons. A dedicated treatment attempting to tag the $\tau $-lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from $\tau ^+\tau ^-$ systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in $Z\gamma $ events using proton–proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of $139\,\text{ fb}^{-1}$. The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.