Officials are warning it is critical to update your Windows computer, in the midst of a global cyber attack.

The threat comes from a ransomware known as Petya, which so far has affected shipping giant Maersk and forced the Chernobyl radiation monitoring system offline.

The government's cybersecurity taskforce, Cert, said Petya only needs one out-of-date computer to infect a whole network.

In March this year Microsoft released a patch for the vulnerability Petya exploits.

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Cert said Petya encrypts files, blocking access to them and demandi6ng the user pay a ransom to get them back.

The taskforce has released an image of what it looks like when Petya is trying to encrypt files.

It said if you see the screen, you should immediately turn off your computer, and do not turn it on again.

An IT specialist should be able to recover your files directly from the hard drive, Cert said.

More than 80 companies in Russia and Ukraine were initially affected by the Petya virus, which disabled computers and told users to pay US$300 ($412) in cryptocurrency to unlock them, according to the Moscow-based cybersecurity company Group-IB.

Europol executive Rob Wainwright said the agency was "urgently responding" to reports of the new cyber attack. Europol also said it was in talks with "member states and key industry partners to establish the full nature of this attack at this time".

The strikes follow the global ransomware assault involving the WannaCry virus that affected hundreds of thousands of computers in more than 150 countries as extortionists demanded $300 in bitcoin from victims. Ransomware attacks increased by 50 per cent in 2016, according to Verizon Communications Inc.

If you think your computer is infected

• If your computer suddenly restarts by itself it may be infected by Petya.
• Immediately turn off your computer.
• If it is a desktop computer, unplug the computer from its power source.
• Contact a technology support worker.

Petya

The latest computer virus has spread very quickly across Europe overnight and could arrive in New Zealand at any time.

It is a type of virus known as ransomware. It encrypts all files on your computer rendering them unusable.

How to avoid being infected

Ransomware does not need to find a vulnerability on your computer to infect, it only needs you to click on a link or attachment in an email to infect your computer or multiple computers.

You can protect yourself by not opening any unexpected or suspicious links and attachments in your email.

Important files should be backed up externally.