Data belonging to 2.5 million Kiwis has allegedly been stolen from the MediaWorks website.

A user posted in an online forum on the dark web about 3am Thursday, saying they "plan to sell this data".

It included people's names, addresses, dates of birth, email and phone contact details, the post said.

The user also claimed to possess some private information, such as questionnaire answers, videos and music materials, and votes relating to popular renovation competition reality show The Block NZ.

A MediaWorks spokesperson said the company became aware of the claims on Friday evening and is now investigating.

"The claims relate to data from website competition entries," the spokesperson said.

"MediaWorks takes data security seriously and the technology team is investigating the potential incident with the support of external experts. Last night all current competition entries were moved to a new secure database."

A spokesperson for the Office of the Privacy Commissioner said it had not yet been notified by MediaWorks of any recent privacy breach.

"We expect they would do this if it were a verified breach," they said.

"As with any breach, MediaWorks would need to investigate to fully ascertain the size and scope of the breach and any impact on its New Zealand clients."

The Office of the Privacy Commissioner's focus in such situations was to provide agencies with advice on how minimise the harm caused to individuals, the spokesperson said.

A later post by the unidentified user provided "samples" of the data, and appeared to include information from individuals in various regions across the country. The latest post had been viewed 25 times in 10 hours, according to the website.