It was the mystery windfall that had Perth motorists and police officers rubbing their eyes.

Two days later, they are still scratching their heads.

For a few mystifying moments on Wednesday night, $100 notes appeared to fall from the sky over the Mitchell Freeway just south of Leederville train station.

Some motorists parked in the southbound emergency lane and ran on to the freeway to cash in on the free payday.

Maylands resident Viswa Kirsz was driving home from a netball game about 9.30pm when she saw cars swerving and pulling over.

The 28-year-old's car was starting to break down, adding to the impression that she had entered the twilight zone.

"My car was breaking down, and as I was pulling into the emergency lane I just saw a flurry of notes," Ms Kirsz said.

"I pulled over, got out of my car and noticed everyone else was running on to the freeway to grab them. We're talking a lot of money. They were $100 bills and there were hundreds of them.

"It was the most surreal thing that's ever happened to me."

Ms Kirsz could not help but get in on the act. But in the end, she decided honesty was the best policy and gave the $200 she collected to police officers. Another woman handed over $700.

"I handed it in because it wasn't my money," Ms Kirsz said. "The police had a notepad and took down my number and name. They seemed rather confused."

The seemingly tall tale was confirmed by police yesterday.

"We can confirm that at about 9pm Wednesday, an amount of money was handed to police officers on the Mitchell Freeway," a spokesman said. "Investigations are continuing to locate the owner of the money."

Once the dollar deluge had blown over, Ms Kirsz had plenty of time to reflect while waiting for the RAC. "It's not as if it just came from the sky," she said. "God knows where it came from. I'd like to know."

The "raining money" phenomenon has happened in other parts of the world in recent months. In Kuwait City in February, thousands of notes filled the air on a busy afternoon, causing chaos as drivers abandoned their cars to grab fistfuls of cash.

In Hong Kong on Christmas Eve, two people were arrested after millions of dollars poured out of a van's faulty back door.