It's the island south of Sydney where every day is like a bucks party.

Frolicking and flapping about, between 50 and 100 Australian and New Zealand fur seals are a permanent fixture of the Five Islands Reserve off the coast of Port Kembla, New South Wales.

For the most part, the rowdy rookery perches on Martin Island, and by all accounts, it's a real gentleman's club.

"It's a bachelor pad," said Dr Rowena Morris, a National Parks and Wildlife Service ranger.

"[They're doing] what most bachelors do. Having fun, hanging out, learning how to fish and swim, catching what they need and [having] social interactions with one another," she said.

The young males have increasingly been using Martin Island to 'haul out' and rest between breeding seasons for several decades, but generally it's not a place where pups and mothering females are spotted.

The closest breeding colony is further south on Montague Island off the coast of Narooma, and on both islands, the seal population is booming, according to Dr Morris.

"It's happening overall in NSW. It's another great conservation story," she said.

Hunted to near extinction

Male fur seals frollic under water at Martin Island near Port Kembla, NSW. ( ABC Illawarra: Sydney Pead )

During Australia's settlement and into the 20th century, fur seals were hunted to near extinction for their fur.

"They have very dense under-fur to keep them warm and on top of that a much thicker hair," said Professor Rob Harcourt, from Macquarie University's Marine Predator Research Group.

"It means that their fur is really fantastic for things like fur coats, so they were hammered in the early times of European settlement.

"Fur seal numbers pre-sealing are estimated at around 500,000. Now in NSW it's between 10,000-15,000," he said.

Professor Harcourt said thanks to conservation efforts since the 1930s, fur seals were dispersing further from their breeding colonies, resulting in more populated haul out spots like Martin Island, where they have more protection from predators.

The Marine Predator Research Group has been tracking seals breeding on Montague Island and another haul out spot at Jervis Bay.

The seals have been found swimming as far north as Newcastle harbour and across to New Zealand, Tasmania and South Australia.

Seal-watching popular with tourists

Along with the increase in the seal population, there has been a rise in interest from tourists.

Abyss Scuba Diving general manager Rachael Fallon, who has run seal diving excursions on Martin Island for about 15 years, said there was more demand than ever.

"I'd maybe do one [dive] a season but the last three years we've been going every second weekend over the period of June until October," Ms Fallon said.

"We're pretty much booked out a couple of weeks in advance. People love it and once they do it once, they'll do it again and again."