A method, for detecting overpressure inside a compartment associated with a gas turbine nacelle, includes: a) measuring one or more predetermined operating conditions nominally affecting the temperature inside the compartment; b) determining a reference temperature for the compartment, corresponding to a nominal reference zone pressure, on the basis of the measured operating condition or conditions; c) measuring the actual temperature inside the compartment, corresponding to the actual compartment pressure; and d) using the measured actual temperature in the compartment and the determined reference temperature for the compartment to assess the gas pressure inside the compartment. A system for assessing the gas pressure inside the compartment caused by an associated pressure system failure includes using a quantified mass leakage flow to assess gas pressure inside the compartment. The quantified mass leakage flow may also be used to detect excessive mass leakage flow by comparing the quantified mass leakage flow with a predetermined mass flow leakage.