When studying diving behaviour, it is important to know whether a species is operating at or close to its maximum physiological capacity, because if it is, it will be less capable of compensating for normal environmental or human-induced fluctuations in its environment. New Zealand (NZ) sea lions Phocarctos hookeri are among the world's rarest pinnipeds with a restricted distribution and abundance to the most southerly limit of their known range, NZ's sub-Antarctic. Female NZ sea lions are the deepest and longest diving of the otariids, foraging further from their breeding rookeries than any other sea lion. In this study, the diving behaviours of 18 female NZ sea lions from Enderby Island, Auckland Islands, were recorded during early lactation over two austral summers, 2003 and 2004. While at sea, sea lions dived almost continuously, spending on average 52.7% of their time submerged (>6 m). The mean dive depth (±se) for all dives was 129.5±5.3 m (range 94.6±1.1 to 178.9±1.6 m). The mean duration of dives was 4.0±0.1 min with an average of 40±2.9% of all dive times spent in the deepest 85% of the dive. Although there was high variation in diving behaviour among individuals, overall, animals were found to be diving beyond their estimated aerobic dive limits on 68% of all dives. Given that female NZ sea lions have a high percentage of dives that are beyond their theoretical aerobic limits, we ask whether this represents a miscalculation in aerobic limits, is it a species highly adapted to carry an anaerobic load or a species that is physically stretched to their limits? A species that is presumably under physiological stress just to maintain its current low static population numbers is also likely to be more susceptible to external impacts and this needs consideration for their management.