The death of a man whose body lay undiscovered in the toilet of a fast food restaurant for three days is to be investigated by the West Australian coroner.

Police were called to Hungry Jack's on Wanneroo Road in the Perth suburb of Westminster on July 18 after the body was found.

It is believed the man died after overdosing.

A report is being prepared for the coroner but the death is not being treated as suspicious.

It is understood the toilet cubicle had a floor to ceiling door with a single vent.

The City of Stirling said health and compliance officers were investigating, in terms of the health and safety at the site.

The store was open for business this afternoon.

The local store manager declined to comment when contacted by the ABC.

Mystery surrounds how body overlooked for three days

When contacted by the ABC, Hungry Jack's head office did not address questions relating to the cleaning schedule of the bathrooms, but issued a short, unattributed statement.

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

"Our thoughts are with all concerned."

Customers at the store today expressed concern about hygiene standards at the fast food facility.

"It's unfortunate, and you have to worry about the QA [quality assurance] and how often they check their facilities," customer Felix Davis said.

"Three days is a pretty long time, poor guy."

Another customer, who gave his name as Rob, said he was concerned the toilets had apparently not been checked for three days.

"Three days is a long time for somebody to be locked in a toilet cubicle and for it not to be cleaned," he said.