A man in his 50s was bitten by a bat carrying Australian bat lyssavirus several months ago, the health service in New South Wales said, EDnews reports, citing AFP.

"We express our sincere condolences to the man's family and friends for their tragic loss," NSW Health said in a statement.

"While it is extremely rare to see a case of Australian bat lyssavirus, there is no effective treatment for it."

The man from northern New South Wales, who has not been identified, was this week listed as being in a "critical condition" in hospital.

Officials said he was treated following the bite and they were investigating to see whether other exposures or factors played a role in his illness.

The virus -- a close relative to rabies, which does not exist in Australia -- is transmitted when bat saliva enters the human body through a bite or scratch.

First symptoms can take days or years to appear.

Early signs of the disease are flu-like -- a headache, fever and fatigue, the health service said.

The victim's condition rapidly deteriorates, leading to paralysis, delirium, convulsions and death.

There were only three previous cases of human infection by Australian bat lyssavirus since it was first identified in 1996 -- all of them fatal.