  Narrow resonances decaying into $WW$, $WZ$ or $ZZ$ boson pairs are searched
for in 139 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy
of $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the Large Hadron
Collider from 2015 to 2018. The diboson system is reconstructed using pairs of
high transverse momentum, large-radius jets. These jets are built from a
combination of calorimeter- and tracker-inputs compatible with the hadronic
decay of a boosted $W$ or $Z$ boson, using jet mass and substructure
properties. The search is performed for diboson resonances with masses greater
than 1.3 TeV. No significant deviations from the background expectations are
observed. Exclusion limits at the 95% confidence level are set on the
production cross-section times branching ratio into dibosons for resonances in
a range of theories beyond the Standard Model, with the highest excluded mass
of a new gauge boson at 3.8 TeV in the context of mass-degenerate resonances
that couple predominantly to gauge bosons.
