The Australian police are investigating how a man's body could have gone unnoticed for nearly three days in the toilet of a popular Australian fast-food restaurant.

Local media reported that the body was discovered on Monday in a locked cubicle at the Hungry Jack's outlet on Wanneroo Road in Westminster, in a suburb of Perth. The police believe the man might have entered the toilet on Friday.

"Hungry Jack's is cooperating with all relevant authorities regarding the matter at its restaurant in Balga, Western Australia," the fast-food outlet said in a statement.

"Our thoughts are with all concerned," it added.

The police could not give any indication of how long the man had been dead before his body was discovered.

The reports said the restaurant was open 24 hours on Saturday and had closed for the weekend at 10pm on Sunday.

"The death is not suspicious and a report will be prepared for the coroner," a police spokeswoman told AFP.

The Australian Broadcasting Corporation said an investigation is underway to find out when the man died and other relevant information.