A Waikato DHB spokesperson told the Herald they hoped to have everything up and running by the weekend.
The outage has impacted all hospitals under the Waikato DHB including Waikato, Thames, Tokoroa, Te Kuiti and Taumarunui.
Outpatient clinics have been cancelled and families trying to reach their loved ones in hospital were urged to contact them on their personal mobile phones where possible due to the hospitals' phone lines also being down.
A significant number of elective surgeries had been postponed and access to patient notes was also limited but was expected to improve over the day, Snee told RNZ.
Former Waikato DHB member Dave Macpherson said they had been warned this could happen if they didn't keep up with what was needed to protect themselves from the type of attack.
"They always knew this sort of thing was likely to happen, but you would think they would have a plan B."
Macpherson had been at Waikato Hospital this morning and said the hospital had reverted to taking notes with pen and paper.
He spoke to one man whose wife was having an elective caesarean that morning and he had to write her name in a black Sharpie pen on the patient wrist band as the scanner was not working.
"It seemed like they were struggling through doing what they could."
He had heard the hospital was using laptops not connected to the network and assumed they might be accessing information from GPs.
Patients turning up to the emergency department were being redirected to other after hours clinics, he said.
Macpherson said the hospital seemed to be very quiet and he had not seen one ambulance arrive in the 45 minutes he had been there.
The DHB has called in the help of external cyber security experts and the national spy agency.
Earlier indications to staff yesterday suggested the IT systems might be down for 24 hours - but the latest advice from the DHB to the Herald indicated it will be at least four days.
A Waikato DHB statement released yesterday said it was "working hard" to get its service back online.
A spokesman for the National Cyber Security Centre (NCSC) - a branch of the Government Communications Security Bureau (GCSB) - confirmed staff were providing support to Waikato DHB following yesterday's attack.
The spokesman said the NCSC's role was to help protect New Zealand organisations of national significance "from advanced, persistent, primarily state-sponsored, cyber security threats".
"We are very conscious that malicious cyber actors can monitor public commentary on an incident and for this reason, while the investigation and remediation efforts are ongoing, we will not provide additional details regarding its cause or the response to it."
Police are also making initial inquiries into the cyber attack, a police media spokesperson said.
