Human bones found in a national park south of Sydney are those of a man but his identity and cause of death is still unknown.

Police said a post-mortem exam on Wednesday determined the remains found in the Royal National Park on the weekend were male but all other tests were inconclusive.

Further tests are expected.

Police believe that the bones could have been there for months if not years. It's also been revealed that the remains were found inside a large plastic bag. 

Police said homicide detectives are expected to lead the investigation after the discovery was made in Sydney's south over the weekend by a motorist who had stopped for a bathroom break. 

Royal National Park: Homicide detectives have been called in after the discovery of bones at the Royal National Park. Police have been searching nearby bushland as forensic experts begin the task of trying to identify the remains. @ebatten7 #RoyalNationalPark #7News pic.twitter.com/1E160sljRy

— 7 News Sydney (@7NewsSydney) September 24, 2018

Officers were called to Sir Bertram Stevens Drive in the Royal National Park following reports that bones were found at the bottom of a ravine just before 5pm on Saturday.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.

With AAP.