  A search for flavour-changing neutral currents (FCNC) processes in
proton-proton ($pp$) collisions at a centre-of-mass energy of 13 TeV with the
ATLAS detector at the CERN Large Hadron Collider (LHC) is presented. The
analysed data collected during the years of 2015 and 2016 corresponds to an
integrated luminosity of 36.1 fb$^{-1}$. A search considering top-quark
pair-production events is performed, with one top-quark decaying through the
dominant Standard Model (SM) mode $t$ $\to$ $Wb$, and the other through the $t$
$\to$ $qZ$ ($q$=$u$,$c$) FCNC channel. The data are consistent with the SM
expectation and the observed and expected upper limits on the branching ratio
of $t$ $\to$ $uZ$ and $t$ $\to$ $cZ$ are set at 95% confidence level
representing an improvement of about a factor 3 compared with the Run-1 data
results from the ATLAS Collaboration.
