A law firm in New Zealand has been affected by the cyber attack gripping the world.

Ransomware known as Petya has so far affected shipping giant Maersk, US drugmaker Merk and the radiation-monitoring at Ukraine's shuttered Chernobyl power plant.

Also infected by the virus is global law firm DLA Piper.

"In response to this incident, DLA Piper globally has shut down all IT infrastructure. Your data is secure and has not been ex-filtrated," Martin Wiseman, country managing partner for DLA Piper, said in a statement to clients.

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The firm in New Zealand can not receive external emails while its IT is shut down, but is still able to send emails.

"While we are operationally integrated with DLA Piper global, our IT systems, with the exception of our email system, are essentially separate," the spokesman said.

Mondelez International has also been struck down the ransomware, but questions to the company for confirmation on whether its New Zealand operations have been affected have gone unanswered.

The government's cybersecurity taskforce, Cert, said Petya only needs one out-of-date computer to infect a whole network.

Cert said there are reports that a 'kill file' can be created that will prevent the ransomware from executing.

The kill file is reported to be called perfc. If people believe their Windows computer has been infected by the ransomware they can implement the kill file by creating a file in c:\windows called "perfc".