All outpatient activity at the Waikato DHB's rural hospitals - including Thames, Tokoroa and Te Kuiti - has also been postponed.
Six of 101 elective surgeries were scrapped yesterday.
Anyone with queries about the urgency of their outpatient appointment should contact their GP, the board said.
Waikato DHB staff along with some Government departments are working to restore the infected systems but it was a "complex process which will take more time to resolve". It had also enlisted the help of external cybersecurity experts.
"We currently have business continuity plans in place to keep our services running into the weekend."
Emergency Departments at all the Waikato DHB's hospitals were open for emergencies only and anyone else was being referred to their GP or local urgent care centre.
Some patients were also being sent to other hospitals around the country. Capital & Coast DHB spokesperson Chas Te Runa confirmed three Waikato DHB patients were being cared for at Wellington Regional Hospital.
Tauranga and Whakatāne hospitals were also getting ready to look after patients that would normally be cared for at Waikato Hospital.
Bay of Plenty District Health Board acting chief operating officer Bronwyn Anstis asked people to use the emergency department for emergencies only.
"Patient care is our top priority and we are doing all we can to assist our colleagues at Waikato DHB at this time."
A Counties Manukau Health spokesperson said they had not taken any patients from Waikato DHB yet, but northern region DHBs were liaising with Waikato around the potential transfer of patients if required.
St John general manager of ambulance operations Stuart Cockburn said St John was monitoring the situation at all Waikato hospitals and working with the board.
St John has a command unit at Waikato Hospital to provide administrative support to ambulance crew and, where possible, was transporting patients with non-urgent condition to other facilities.
Patients requiring emergency care would continue to be taken to an emergency department for assessment and treatment, he said.
Hamilton woman Deborah Cuming took her father to Waikato Hospital for an appointment at the eye clinic this morning and was impressed by how staff were dealing with the situation.
Cuming said the parking barriers were up and signs at the entrance said there was no charge.
Once inside the hospital, people were orderly and staff were relying on hard copies of patient files. They were taking notes by hand, which would be loaded into the system at a later date, she said.
The only difference was that the optometrist who treated her father had been unable to access previous scans to do a comparison.
He told her the team had found their feet after a chaotic day yesterday.
"The attitude among the staff and patients was awesome."
While she was there, a woman arrived with an armload of cakes to thank the staff for their hard work.
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 after yesterday's attack.
The spokesman said the centre's role was to help protect New Zealand organisations of national significance "from advanced, persistent, primarily state-sponsored, cybersecurity threats".
"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 inquiries into the cyber attack, a police spokesperson said.
Anyone needing help is advised to check the Waikato DHB's Facebook page or, from Wednesday afternoon, a new number - 0800 276 216 - will also be available for external enquiries.
