Waikato DHB has admitted there's a real threat some people's personal information may have been breached, as a result of the cyber attack.
The DHB is still working to get its systems back online - something it has described as "more complex" than it thought.
Chief executive Kevin Snee says he understands there may be some anxiety about the fate of data and information held by the DHB.
The DHB has been given the services of the Government Communications Security Bureau to sort out the problem.
"There is a chance they will have taken data that was on the system, and exfiltrated the data - put it somewhere else, usually in the dark web for later use," Health Minister and GCSB Minister Andrew Little told Checkpoint.
The widespread nature of the attack revealed "a high degree of exposure," Little said. A review will establish exactly what happened.
"You would expect in this day and age for a sophisticated organisation like DHB running a series of health services, there would be adequate protections around those sorts of things."