•A long duration low amplitude signal segment distinguishes intermoult lobsters.•A 7 MHz centre frequency transducer generates the distinguishing signal segment.•Pre-moult structural changes beneath the exoskeleton explain changed response.•A foundation is indicated for an NDT based ultrasonic crustacean pre-moult detector.•Valuable insights of acoustic interactions with the lobster exoskeleton are presented. This study examined the novel application of non-destructive testing (NDT) ultrasound for detecting approaching moult in juvenile tropical rock lobsters (TRL), Panulirus ornatus. TRL juveniles are highly cannibalistic in culture, with up to 25% of lobsters lost to cannibalism at each moult. A pre-moult sensor could prove pivotal as moulting lobsters are the primary victim of cannibalistic predation and a reliable indication of pre-moult could trigger a suitable intervention measure. A series of structural changes take place beneath the exoskeleton in preparation for moulting, which leave little indication of pre-moult on the lobster’s external surface. An opportunity exists for penetrative pre-moult detection via ultrasonic acoustic stimulation through the exoskeleton. NDT ultrasound is proposed as a possible pre-moult sensing technology. NDT ultrasound is comparatively low-cost, a requirement for replicated deployment to industry. This study explores the acoustic response from five NDT ultrasound transducers by scanning the dorsal carapace of juvenile TRLs. A small diameter (13 mm) transducer with a 7 MHz centre frequency produced and measured a low amplitude, long duration signal segment in intermoult lobsters that was not present in pre-moult lobsters, forming a basis to distinguish intermoult from pre-moult lobsters. It is proposed that muscle detachment and growth of the new integument and a separating membrane disrupt sound conduction beneath the exoskeleton in a similar fashion to delaminated joint detection methods used in industry. These results show promise that a pre-moult sensor may be able to be based on NDT ultrasonic technologies and provide guidance for refining transducer specifications for further research.