At least 5 Australian businesses affected by ransomware

The number of Australian businesses reported to be affected by a global cyber attack has risen to five, as authorities continue investigations into the source of the so-called 'WannaCry' ransomware. Australia appears to have been sheltered from the worst of the attack, which knocked out the computer systems of hospitals, rail networks and schools in Europe. But experts are warning Windows users not to be complacent. Preeti K McCarthy reports.

A programmer shows a sample of a ransomware cyberattack on a laptop in Taipei Source: Taiwan (AAP)

As Australians returned to work on Monday morning, the impact of the cyber attack was revealed.

The government says it's working with five affected companies.

The minister in charge of cyber security, Dan Tehan [TEE-an], says it's an important reminder to keep software up to date.

"I think it is a real wake-up call for all of us. My hope is what it will lead to is more urgency and more focus from departmental and agency heads at the government level, from boardrooms at the business level and from individuals and families of the need to make sure you're doing everything you can to keep yourselves cybersecure."

The WannaCry attack spread across Europe on Friday evening, shutting down company computer systems and vital government services.

Worldwide it is estimated to have locked 200,000 computers.

Its effects were felt in Australia on Saturday morning.

Malcolm Turnbull's cyber security advisor, Alastair MacGibbon, says that timing might have prevented a similar breakout here.

"It was early morning Saturday for us, and that meant the bulk of us weren't at our keyboards. We weren't able possibly to ingest the threat vector which was probably an email that contained this malicious code in it, or a link to malicious code, and as a consequence we were probably better protected than our European cousins."

Ransomware attacks by locking a computer and encrypting all the data.

Users are then asked to pay the hackers a financial "ransom" to regain control.

But Mr MacGibbon says businesses are better off relying on their backups if they have them - and that there's a chance someone will find a way to undo the encryption without paying.

"There is a possiblity that decryption could occur. We clearly don't advocate paying criminals because we don't do it offline and we shoudn't be doing it online, and that's the best advice I can give."

The hack exploits a flaw in Microsoft Windows operating systems, and the government is urging all Australians to update and their computers and computer security systems.

Microsoft has confirmed the hacking method used was built by the National Security Agency in the United States, and stolen from the agency in April last year.

Microsoft president Brad Smith says the incident shows that governments should tell software companies about security flaws, rather than keeping silent about them.