A third party file-sharing service used by the Reserve Bank of New Zealand to share and store sensitive information had been illegally accessed, the Wellington-based bank said in a statement on Sunday.

Governor Adrian Orr said the breach has been contained. The bank's core functions "remain sound and operational", he said.

"We are working closely with domestic and international cybersecurity experts and other relevant authorities as part of our investigation and response to this malicious attack," Orr said.

"The nature and extent of information that has been potentially accessed is still being determined, but it may include some commercially and personally sensitive information."

The system had been secured and taken offline until the bank completes its initial investigations.

"It will take time to understand the full implications of this breach and we are working with system users whose information may have been accessed," Orr said.

The bank declined to answer to emailed questions seeking more details.

It's unclear when the breach took place or if there were any indications of who was responsible, and in what country is the file sharing service based.

Several major organisations in New Zealand have been the target of cyber interference in the past year, including the New Zealand Stock Exchange, which had its servers knocked out of public view for nearly a week in August.

Dave Parry, professor of computer science at Auckland University, told Radio New Zealand that another government was likely behind the bank data breach.

"Ultimately if you were coming from a sort of like criminal perspective, the government agencies aren't going to pay your ransom or whatever, so you'd be more interested probably coming in from a government-to-government level," Parry said.

Australian Associated Press