The cyber attack that's gripping the world is also affecting some New Zealand firms.

Ransomware known as Petya has hit international law firm DLA Piper.

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

DLA Piper country managing partner Martin Wiseman said the firm in New Zealand can not receive external emails while its IT is shut down, but is still able to send emails.

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

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

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The government's cybersecurity taskforce, Cert, said Petya only needs one out-of-date computer to infect a whole network.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