An unidentified man gave the key to a worker at a chapel in Sydney's infamous Kings Cross red light district on Friday evening, police said.

Police appealed for help earlier on Friday after the original treasure from the French Revolution was reported missing.

The key - on loan from Madame Tussaud's wax museum in London - was removed from the wall using wire-cutters.

The Madame Tussaud wax museum offered a reward of 3,000 Australian dollars (US$1,950) for the recovery of the cast-iron key, which disappeared last Saturday.

"I felt sick when I heard the news from a guard," said Madame Tussaud's general manager, Vicky Brown.

"It's not something you expect to get stolen."

The key is about 10 inches long, weighs more than four pounds and has a distinctive crucifix cut out of its wrought iron work.

Key rescued at beginning of revolution

The Bastille was built in 1369 by Charles V to defend Paris against the English.

It was later used as a state prison before being stormed on July 14, 1789 at the start of the French Revolution.

The key was collected the following day by Madame Tussaud, who worked as an art tutor at the royal palace in Versailles.

She was later arrested by revolutionary forces but escaped execution after agreeing to make death masks of guillotined aristocrats.