The Waikato District Health Board is working to contact patients whose data has got into the wrong hands, chief executive Kevin Snee says.
It was revealed on Wednesday information hacked during a cyber attack six weeks ago has ended up on the dark web.
The DHB's hospitals and services were severely disrupted following the attack - with some having to seek treatment at other hospitals throughout the country.
Snee told a news briefing on Thursday the DHB is working with privacy experts to resolve the matter.
But he could not say how many patients had been affected by the most recent data dump on the dark web.
Snee said it was "certainly more than dozens" of patients affected but could not provide specific numbers.
"There's a process that we need to work through that we're working through with privacy experts and the Privacy Commissioner, and as soon as we work through that for staff and patients we will then contact."
He said patients will be notified as soon as an assessment process is finished.
"We appreciate this may cause concern for patients and staff so we're working closely with the Privacy Commissioner and other privacy experts. We will offer advice and support to anyone affected on how they can protect themselves and their data going forward.
"It's important that we're able to notify anyone and provide advice on what steps they should.
"The DHB continues to treat this incident very seriously and has allocated significant resources to manage the incident response."
Once all services were back online, an independent probe into the cyberattack would take place, he said.
"It wouldn't be appropriate to talk about our systems. That will be subject to an inquiry at a later stage… My focus is on [the] restoration of services.
"We're grateful to the people of Waikato for their understanding during the remediation and also to staff for the tireless work who brought the services back," said Snee.