Auckland Mayor Phil Goff's family email account has allegedly been hacked, and thousands of emails stolen.
In a statement, Goff's chief of staff Nirupa George said the mayor assumed the service provider's platform was secure.
George said while the authenticity of the hack had not been confirmed, the mayor had stopped using the account and taken advice from experts.
Police and relevant agencies are said to be investigating the matter.
An internet safety expert says the news shows New Zealanders need to be more serious about cyber security.
Technology Director for cyber security consultancy Cyber Toa, Tony Grasso, said people need to understand the threat online.
"People don't realise that physical threat and electronic threat are very different. So New Zealanders still feel like it's really safe here. So our security here is poor, because people just don't appreciate the risks.
"But actually with electronic threat you're as susceptible as anyone - this isn't the stuff of fantasy, this stuff goes on all the time, it's very common."
Grasso said while the global reach of the internet means threats could come from anywhere, there is an active hacker community in New Zealand who might target locals.
"One of the biggest threats is local hackers ... they do a lot of this stuff here, it's quite prevalent in New Zealand.
"You do get foreign hackers that do these for bounties, through their governments, but I think it's likely local."
Police are investigating the theft of Goff's emails.