The accurate prediction of the dynamic responses of fish cages under extreme sea states is of vital importance for the structural reliability of marine aquaculture infrastructure. The present study investigates the hydroelastic behaviour of flexible fish pens under different extreme wave conditions with specified recurrence periods. Initially, environmental contours for extreme wave conditions were derived using the direct Inverse First Order Reliability Method (IFORM), by fitting the generalised extreme value (GEV) distribution to the declustered storm peaks and extrapolating to conditions with specified recurrence periods. The wave condition contours are then mapped to the response space of the fish cage according to the Norwegian standard NS9415 using finite element method (FEM) numerical simulations. The research results indicate that significant differences in the wave responses of the fish cage occur under severe non-severe extreme wave conditions. The study also examines the reliability and serviceability of the fish cage under these extreme sea states. It is found that while the fish cage exhibits a substantial hydroelastic response at low frequencies and high energy, this response is reduced under severe sea conditions. The techniques and findings provide valuable insights for the reliability analysis of marine aquaculture infrastructure.