Australian Sea Lions are sexually dimorphic, with adult males reaching 1.25 times the length and 2.5-3.5 times the weight of adult females. Very little information on sizes of adult males is available, and some values in the literature may be overestimates. Adult males reach lengths of 1.8-2.5 m and weigh 180-250 kg. Females are 1.3-1.8 m in length and weigh 61-105 kg. At birth, pups are approximately 60-70 cm long and weigh 6-9 kg (Lowther and Goldsworthy 2011). Pups are dark chocolate brown to charcoal in colour at birth and lighten to a smoky grey before becoming brown. The pelage pattern of pups up until their moult at around 4 months of age is highly variable.

Females become sexually mature at 4.5-6 years of age and males at six years or more. The mean age of breeding females is 11 years (McIntosh 2007). The oldest breeding female recorded was aged 24 years, while the maximum longevity recorded is 26 years for females and 21.5 years for males (McIntosh 2007). Age-specific survival probabilities are high (0.98) after six years of age and are similar for males and females (McIntosh 2007).

Australian Sea Lions are unique among pinnipeds, being the only species that has a non-annual breeding cycle, with intervals between pupping seasons of ~17–18 months (Ling and Walker 1978, Higgins and Gass 1993, Shaughnessy et al. 2006, Goldsworthy et al. 2014). Furthermore, breeding seasons are protracted in duration (4-9 months), and occur asynchronously across the species range (breeding can occur at any time of the year) (Shaughnessy et al. 2006, Goldsworthy et al. 2014). Asynchronous breeding is maintained through extremely low rates of interchange between colonies by adult females, as demonstrated by genetic studies that indicate extreme population sub-structuring of mitochondrial DNA lineages (maternally inherited), even for those separated by short distances (Campbell et al. 2008, Goldsworthy and Lowther 2010, Lowther et al. 2012). The evolutionary determinants of this unusual reproductive strategy remain enigmatic (Goldsworthy et al. 2009). Given the high level of genetic subdivision at the breeding colony scale, individual breeding colonies are considered subpopulations in this species (Campbell et al. 2008, Lowther 2010, Lowther et al. 2012).

The low synchrony of births within a breeding season requires males to mate-guard individual females until they come into oestrus around 7–10 days following the birth of the pup. Males are therefore sequentially polygynous, mate-guarding individual females until the male is compelled to go to sea and forage, after which he returns and repeats the strategy. Males defend their female(s) with guttural clicking, growling and barking vocalizations, posturing, and by fighting with rivals.  Like most pinnipeds, there is a 4–6 month period of delayed implantation of the blastocyst following conception. This leads to a prolonged (active) placental gestation of up to 14 months, the longest for any pinniped (Gales and Costa 1997). The annual pregnancy rate of mature females is ~71 % (Higgins and Gass 1993).

Pups are continuously attended for the first 9–10 days after birth by their mother, after which adult females alternate between foraging trips to sea and nursing bouts ashore. Females nurse their pups for 15–18 months, usually weaning them before giving birth again, however females will nurse offspring for three or more years if they do not pup in the subsequent breeding season or their new pup dies. Adult female Australian Sea Lions behave aggressively toward pups that are not their own. Pups will play at the shoreline and in tide pools while their mothers are away, and following their postnatal moult, will actively swim on their own. Pups commence their own foraging trips at a young age and by six months can occupy part of adult female home range and explore adult foraging habitat at least eight months prior to weaning, allowing them to learn the location of suitable foraging habitat and the skills required to hunt successfully (Lowther and Goldsworthy 2012).

Australian Sea Lions restrict their foraging activities to continental shelf waters, with juveniles, adult females and adult males rarely exceeding depths of 90, 130, and 150 m, respectively (Goldsworthy et al. 2010). The maximum recorded dive depth for an adult male is ~250 m (Goldsworthy unpublished data). The maximum recorded foraging ranges of juvenile and adult female seals are 118 and 190 km, respectively (Goldsworthy et al. 2010). Adult males range much further and have been tracked up to 340 km from their colony. There is marked within and between-colony variability in the foraging behaviour of juveniles, adult females, and males (Goldsworthy et al. 2009, Goldsworthy et al. 2010, Lowther and Goldsworthy 2011, Lowther et al. 2012). Foraging trips to sea are relatively short compared to other otariids (mean 1.1 day and maximum 5.1 days in juveniles; mean 1.2 day and maximum 6.2 days in adult females; mean of 2.5 days and maximum 6.7 days in adult males; Higgins 1993, Higgins and Gass 1993, Lowther and Goldsworthy 2011, Kirkwood and Goldsworthy 2013). Australian Sea Lions are benthic, diurnal foragers that routinely transit to foraging locations by swimming along the bottom and dive continuously while at sea and forage at all times of day (Costa and Gales 2003). During dives they minimize the time spent during the descent and ascent phases in order to maximize foraging time on the seabed. Individual dives rarely exceed eight minutes in duration enabling animals to perform around 10–11 dives per hour (Kirkwood and Goldsworthy 2013). Australian Sea Lions are fast, powerful swimmers and often “porpoise” out of the water when moving rapidly at the surface.

The diet of the Australian Sea Lion is poorly understood. Dietary information available is based on limited scat (faeces), digestive tract (autopsied dead animals), and regurgitate analyses (Gales and Cheal 1992, Ling 1992, McIntosh et al. 2006), some crittercam footage (Fragnito 2013), and analyses of prey DNA recovered from faeces (Peters et al. 2014). Cephalopods appear to be a key component of the diet, and include Octopus (Octopodidae), Squid (Loliginidae), and Cuttlefish (Sepiidae) species. Key fish taxa include Leatherjackets (Monacanthidae), Wrasse (Labridae), Flatheads (Platycephalidae), Perch (Sebastidae, Serranidae), Cods (Moridae), Mullets (Mullidae), and Nannygai/Redfish (Berycidae), Whiting (Siikginidae), Rock-ling, and Stingaree /Fiddler Ray (Urolophidae, Rhinobatidae). Small pelagic fish including Jack Mackerel, Yellowtail Scad, and Australian Sardine, have been recorded in the diet, but are not common (McIntosh et al. 2006, Peters et al. 2014). Crustaceans have also been recorded in the diet and include crabs (Stone Crab), prawns, and Rock Lobster (Jasus edwardsii) (McIntosh et al. 2006, Fragnito 2013). Crittercam data indicate that diet and feeding behaviour can vary markedly between individual animals (Fragnito 2013).

Predators of Australian Sea Lions include Great White Sharks (Shaughnessy et al. 2007) and presumably Killer Whales (Ling 2002).

This species is protected in Australia by State Conservation agencies under numerous independently enacted state laws, the earliest dating back to 1889. The Environment Protection and Biodiversity Conservation Act, 1999 provides protection for all pinnipeds in Australia. The Australian Sea Lion was listed as a Threatened species in the Vulnerable category under that Act in 2005. It is listed as a protected species (rare) in South Australia under the National Parks and Wildlife Act, 1972. In Western Australia they are protected under section 14 of the Wildlife Conservation Act, 1950 and are listed as specially protected in Western Australia under the Wildlife Conservation (Specially Protected) Fauna Notice 2005.

Between 2010 and 2012, the Australian Fisheries Management Authority (AFMA) introduced a range of management measures into the shark gillnet component of the Gillnet Hook and Trap (GHAT) fishery to mitigate the impacts of bycatch mortality on Australian Sea Lion populations off South Australia (AFMA 2010). This followed research that integrated an on-board independent bycatch observer program on gillnet vessels, and an extensive Sea Lion satellite tracking and spatial modelling program (Goldsworthy et al. 2010). The management measures introduced include spatial closures (most between 4–10 nm) excluding the fishery around all Sea Lion colonies off South Australia, and bycatch trigger limits that place a cap on the total numbers of Sea Lions that are permitted to be caught within areas of the fishery, which if exceeded result in extended (18 month) fishery closures (AFMA 2012). Given there is 100% observer coverage in this fishery, mostly through electronic monitoring, there is high compliance and most Sea Lion bycatch is now reported in fishery logbooks (AFMA 2013). Additional management measures, such as switching gear to hook and line, are being considered. Bycatch of Australian Sea Lion pups and juveniles in Rock Lobster pots has been largely mitigated through the introduction of Sea Lion excluder devices in the Western Australian and South Australian Rock Lobster fisheries.