Waikato District Health Board has been crippled by a virus shutting down its network of 3000 computers and affecting 5600 staff.
The outage affecting the Waikato Hospital and the DHB's hospitals in Thames, Taumarunui and Te Kuiti is expected to last 24 hours though Waikato DHB incident controller Kevin Harris cannot guarantee the timeframe.
"We have been told by the Information Services (IS) team that the expected outage would be 24 hours and we are confident with their assessment," Harris said.
Harris said there would no problem delivering critical care for patients and that urgent radiology and laboratory testing would be available.
"We are monitoring the situation around surgery and outpatient clinics and we are now considering our next steps around these," he said.
Emergency departments were coping with the outage though Harris said low numbers presenting were helping with the situation.
Patients with appointments scheduled within the next 48 hours would be contacted directly if they could not be kept by the DHB.
Harris said the outage was reported at 2am this morning and is believed to be a well known virus known as 'conficker'.
"We know it attacks the system rapidly and searches for passwords, anyone with an easy password would find themselves exposed," he said.
"We have been developing and implementing plans though getting messages to staff has meant a return to old methods," he said.
Media and communications manager Mary Anne Gill said an urgent memo was distributed via "runners" from early morning instructing staff about the outage.
"We have also faxed information to GPs and other service providers asking them not to send patients to the hospital unless in emergency situations," Gill said.