This paper proposes a theoretical framework modelling the effect of simultaneous sectorial shocks on the banking system resilience, in presence of overlapping portfolio risk. A sectorial shock decreases the value of banks' assets and solvency. We show that beyond a determined threshold, the regulator (central bank) shall intervene in order to avoid that simultaneous sectorial shocks lead to bankruptcies in a cascading process. The model is simulated in the context of the Tunisian banking system for a variety of extreme macroeconomic conditions (economic recession and inflation). The simulations show that the simultaneous shocks to industry, commerce and real estate sectors might lead to the bankruptcies of one to 10 banks among the 23 Tunisian banks via the cascading contagion channel, in the absence of a prompt and adequate intervention by the regulator.