  A search for pair-produced vector-like quarks decaying into a $Z$ boson and a
third generation quark using 2015 and 2016 $pp$ collision data at
$\sqrt{s}~=~13~$TeV collected by the ATLAS experiment at the Large Hadron
Collider, corresponding to an integrated luminosity of $36.1~$fb$^{-1}$, is
presented. No significant excess over the Standard Model expectation was found
so lower exclusion limits at 95% confidence level were set on the vector-like
quark masses at 1030 (1210) GeV for singlet (doublet) $T$ and 1010 (1140) GeV
for singlet (doublet) $B$.
