Health authorities are about to tell more than 4000 people their personal information was breached and shared on the dark web after a damaging cyberattack.

Waikato District Health Board said where possible, it would be in direct contact with the roughly 4200 affected people "from early next week".

The health board said it was reviewing data disclosed on the dark web after the cyber attack in May.

"The DHB continues to treat this incident very seriously," the health board said in a statement this afternoon.

Waikato DHB said it worked with Inland Revenue and Waka Kotahi NZTA to implement security measures to protect impacted personal information.

"We regret that the incident has occurred and apologise for any distress or concern caused," the DHB added today.

"The DHB has worked to securely restore its systems and return healthcare services to full operation to meet our community's daily needs."

The attack left the DHB having to employ hundreds of additional IT experts as it attempted to rebuild its systems.

The outage affected Waikato Hospital and outlying hospitals in Thames, Taumarunui, Tokoroa and Te Kuiti.