AUSTRALIA may have missed the worst of the worldwide cyber attack but small businesses are still at risk of being infected by the WCry ransomware.

Special adviser to the Prime Minister on Cyber Security Alastair MacGibbon said the first thing Australians should do when they get in to work on Monday is update their Microsoft software.

They should do this before they open their emails or other programs.

This is because the virus may be spreading via email attachments or trusted websites, Mr MacGibbon told ABC, and if you don't open or click on to the infected sites then you won't be impacted.

He said it was unclear so far how it had started but ransomware generally spread via a few different methods.

This includes through spear phishing, which spreads the virus through an email that appears to be from an individual or business that you know.

You may also download the virus simply by visiting a website you already trust.

The virus also has a wormlike features that looks for other vulnerable systems once it's embedded in your computer, which means it can spread to other computers in a network.

While Mr MacGibbon said Australia looks to have missed the worst of the attack as it didn't seem to have infected government agencies or critical infrastructure, people shouldn't be complacent.

He said people should be regularly updating their software, and not just when there is a threat.

Generally a popup will appear if there is an update available but you can also force the computer to check for updates.

Mr MacGibbon said people should also be backing up their data on a USB or disk that is kept separate and not connected to their computer.

So far the massive cyber attack has hit 200,000 people in at least 150 countries including an Australian business.

The ransomware locks down computers and has been demanding payments of $US300 ($AU406) to $US600 ($AU812) to restore access.

Initial reports said just one Australian business had been impacted, but Cyber Security Minister Dan Tehan confirmed two others were also hit in an interview with the ABC this morning.

The malware attack first emerged on Friday night but there were concerns of more disruption as workers switch on their computers this morning.

In the UK, the National Health Service has been forced to cancel operations today within its hospitals after computers used to share patients' test results and scans with doctors remain frozen.