Waikato District Health Board's entire IT service is down due to a major cyber attack affecting all clinical services and forcing patient appointments to be cancelled.
"We have engaged external assistance to address a cyber security incident affecting our Information Services environment," the DHB said in a Facebook post this morning.
Government agencies have been advised and an investigation is under way, the DHB said.
They said they are working to resolve the problem "as soon as possible".
"We are uncertain how long it will take to resolve this situation, but we are working hard to get our services back online."
All hospital's under the DHB - including Waikato, Thames, Tokoroa, Te Kuiti and Taumarunui - have been affected "to varying degrees".
"We want to assure the public that all inpatients are continuing to receive quality care from our staff," the DHB said in its post.
"Unfortunately this has meant some outpatient clinics may be cancelled as a result of this outage. Those affected will be contacted to rebook their appointments."
The DHB is encouraging the public to keep Waikato Hospital's Emergency Department for emergencies only.
"This is to ensure we can continue to provide critical services to our patients," the post said.
If you need immediate or urgent help, please continue to call 111. If it is not an emergency, please phone Healthline on 0800 611 116, visit your GP or local urgent care centre.
The landline phone services are also currently down, including the main Waikato DHB number.
For those trying to contact their loved ones in hospital, please consider using personal mobiles where possible, the DHB said.