Cyber experts in New Zealand believe hackers have used malicious attachments or links in emails to hack computer systems around the world.
Computer systems at private companies and public organisations - including hospitals - were attacked on Friday in an apparent extortion plot.
The government's National Cyber Security Centre say they're aware of hackers encrypting malware and demanding ransoms to have data restored.
Online maps appear to show computers in New Zealand have been affected by the malware but those infections are unconfirmed.
Neither the NCSC or National Computer Emergency Response Team have received any reports that Kiwis were targeted in the ransomware attack.
More than 75,000 computers have reportedly been affected in 99 countries around the world, hitting Russia, Ukraine and Taiwan the hardest according to cybersecurity firm Avast.
CERT believes a phishing email with malicious attachment or link was used to penetrate the Windows operating system with the virus, called WannaCry.
"Once a single computer in a network is affected with WannaCry, the program looks for other vulnerable computers in the network and infects them as well," the organisation said.
The malware is also known as Wana DecryptOr, WannaCryptor or WCRY and uses a file extension .wcry.
The ransom was initially $NZ430 but is believed to have doubled over time, CERT understands.
An NCSC spokesman told NZ Newswire the ransomware exploits a known Windows vulnerability, and advice had previously been provided to customers on how to address the issue.
"We are also working with CERT NZ to provide information on how individuals, small businesses and operators of larger systems can reduce their vulnerability to ransomware attacks," he said.
At least 16 British National Health Service organisation were among those targeted, crippling computer systems and telephone lines and causing delays.
Companies including US delivery company FedEx and the Spanish telco Telefonica were also affected.