It crippled Britain's health system - stroke victims were unable to undergo urgent surgery because their scans could not be accessed - and affected other businesses around the world.

Tens of thousands of computer were "locked" - the screens displaying the telltale ransom demand. The devastating cyber attack had, so far, spared Australian targets, but authorities did not expect that to last.

Local experts on Saturday scrambled to ensure hospitals and other public facilities did not fall victim to the massive ransomware operation, which had seen patients turned away and operations cancelled in Britain.

Prime Minister Malcolm Turnbull's cyber security adviser, Alastair MacGibbon, said there were no confirmed incidents in Australia as of Saturday, but warned it was likely some computer systems would eventually be hit.

"Statistically speaking, you'd have to make the assumption that some businesses in Australia will be affected," he told Fairfax Media.

While details were unconfirmed, the consensus of the global cyber security community was that the attack's origins lay in a "cyber weapon" called Eternal Blue first developed by the US National Security Agency.

A mysterious, Russian-linked group called TheShadowBrokers last month claimed to have stolen the hacking tool, which may then have been acquired by another cyber gang and unleashed in Friday's onslaught.

The attack has prompted renewed debate about the risks caused by governments' spying activities.

"We don't know for sure who is actually responsible right now," said Melbourne University expert Suelette Dreyfus. "What we do know is that the cyber security holes were weaponised by the NSA.

"If spy agencies know about major software manufacturers' security holes, and don't tell the manufacturer in a timely fashion, there is a risk it might be leaked. Then it's not only the spy agencies that can break in, it's also criminals."

Ransomware and other malicious software is estimated to cost the Australian economy about $1 billion a year, according to the PM's assistant minister on cyber security, Dan Tehan.

This particular attack used malicious emails to encrypt a computer's data and demand payment of hundreds of dollars to decode the data.

More than 75,000 computers in 99 countries were compromised by the ransomware, Bloomberg reported. Targets reportedly included Russian telecommunications services, FedEx in the US and businesses in Japan, Spain, Turkey, the Philippines and others.

Documenting the European spread of the attack, social media users at a Frankfurt train station and a Milan university posted photographs of screens displaying the telltale ransom demand.

In Britain, a National Health Service staffer told the BBC the intrusion had caused "absolute carnage", with stroke victims unable to undergo urgent surgery because their scans could not be accessed.

"Patients will almost certainly suffer and die because of this," the staffer said.

Experts said malicious software often metastasised in an uncontrolled fashion once it was released on a particular target.

"You start in the UK and it just accidentally spreads, if nothing else," Mr MacGibbon said. "It might just be a European criminal group going after European targets, but these things spread, by their nature."

Last month, TheShadowBrokers posted an English-language letter to US President Donald Trump on the website Medium, accusing him of abandoning his base, back-tracking on Obamacare and launching airstrikes on Syria.

"Respectfully, what the f--- are you doing?" the letter began, before detailing a number of grievances with Mr Trump. The group claims it is not Russian.

In Australia, the proliferation of malicious attacks, including last year's census debacle, has focused attention on the quality of the country's protections against online criminal gangs and other nation states.

A cyber security drive, led from the very top of government, has seen Mr MacGibbon in daily contact with chief executives and heads of Commonwealth departments.

"We have political leadership now in this country that actually cares about this. The Prime Minister is highly interested in these types of matters," Mr MacGibbon said.

"I would tell you that we are directionally correct, but I will never tell you that it's mission accomplished."

Ransomware attacks were an everyday occurrence, Mr MacGibbon said, meaning there were likely incidents underway "this very second". Government systems were just as liable to be attacked as businesses.

"It will be that weird place that you didn't expect it to pop up," he said.

This attack targeted Windows operating systems that had not yet been updated with the latest "patch" by Microsoft, released earlier this year.

"If you haven't downloaded the latest patches, you're potentially at risk," Dr Dreyfus said. "You are a sitting duck."

- with agencies