Victoria Police has taken the extraordinary step of immediately cancelling all fines issued by speed and red-light cameras hit by a computer virus.

Acting Deputy Commissioner Ross Guenther made the announcement on Friday afternoon.

About 55 cameras were affected by the the WannaCry ransomware virus between June 6 and June 22.

The cameras, most of them in inner-city Melbourne, issued 590 speed and red-light fines during that time.

Mr Guenther said drivers would soon receive letters noting that those fines had been cancelled.

Despite the cancellation, Mr Guenther said he had confidence the cameras had been correctly issuing fines.

He decided to cancel the fines in the interest of community confidence in the camera system, he said.

"I cancelled the fines because I think it's important the pubic has 100 per cent confidence in the system," Mr Guenther said.

"My advice is during the period the cameras were operating correctly and were not impacted by the virus. I'm confident in the advice I've been given that the fines would stand."

Typically ransomware spreads by people unwittingly opening emails, clicking on unsafe links or opening attached documents infected with a malware.

But the WannaCry developers have taken advantage of an old Windows exploit (a hole in the code) that meant they could remotely access computers and install their encryptor, allowing the virus to attack networks across the world.

The virus typically locks up infected computers and demands a ransom – payable in Bitcoin – to unlock them.

But Mr Guenther said it was his understanding that no ransom demands had been made.

More to come.