A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the $$Z$$ boson mass, jets tagged as originating from $$b$$ -quarks and missing transverse momentum. The analysis is performed with proton–proton collision data at $$\sqrt{s}=8{\hbox {\ TeV}}$$ collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb $$^{-1}$$ . No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ( $$\tilde{t}_2$$ ) followed by the decay to the lighter top squark state ( $$\tilde{t}_1$$ ) via $$\tilde{t}_2 \rightarrow Z\tilde{t}_1$$ , and for $$\tilde{t}_1$$ pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino ( $$\tilde{\chi }^0_1$$ ) is the next-to-lightest supersymmetric particle and decays producing a $$Z$$ boson and a gravitino ( $$\tilde{G}$$ ) via the $$\tilde{\chi }^0_1 \rightarrow Z\tilde{G}$$ process.