Waikato District Health Board has revealed an additional 4200 patients had their data shared on the dark web as a result the cyber attack on the DHB's systems in May.
The district health board said it has been working to identify which patients had their data breached during the attack, with roughly 4200 more patients added to the list on top of those previously known about.
Over the past few months, the district health board has been gradually working to unpack the impact of the ransomware attack which crippled its IT and communication systems across its five hospitals.
The district health board said it plans to notify the patients affected from the beginning of next week.
"We regret that the incident has occurred and apologise for any distress or concern caused," a spokesperson said.
"The DHB has worked to securely restore its systems and return healthcare services to full operation to meet our community's daily needs."
Information relating to 243 patients was released on to the dark web in July after the DHB refused to pay the hackers.