Staff at Waikato Hospital have been told management have no idea when a massive IT outage will be fixed.

Health NZ spokesperson Stephanie Doe said a "network connectivity issue" had cut access to a number of applications used in Waikato hospitals.

This included Waikato, Thames, Te Kūiti, Tokoroa and Taumarunui.

"Where clinical systems are impacted, hospitals are reverting to manual processes to support continuity of care."

She said hospitals were prioritising urgent care and impacted patients would be contacted directly. Patients who have not been contacted should attend their appointments as normal.

She said the health agency was working to resolve the issue as quickly as possible.

RNZ has seen correspondence to staff which said paper-based request systems must be used in the radiology department.

Theatres will continue for urgent and prioritised care, while the laboratory will be prioritising critical tests. Most outpatient clinics have also been deferred with clinical oversight.

The correspondence said the impacts were varied across services, depending on whether applications were hosted on local servers.

It also said the timeline to resolve the issue was "unknown."

Further correspondence advised staff there was a district-wide issue affecting the integrated software used, which was being investigated.

While it was being looked into and resolved, staff were told to "maintain all clinical documentation using paper-based notes".

"Staff should also continue to record and manage all patient contacts in accordance with the current paper documentation processes."

A further update would be provided as soon as the issue was rectified and "normal system functionality has been restored."

In the meantime, they were asked to ensure all patient documentation is completed and maintained on paper; all relevant patient contacts and interventions are recorded through paper-based process; and any urgent concerns or issues are escalated through the usual channels.

Staff were thanked for their patience and cooperation while it was being resolved.

After another IT outage to hospitals in the upper North Island earlier this year, Auckland University computer scientist Dr Ulrich Speidel told RNZ the country's health IT systems were vulnerable due to decades of neglect.

"That dates back even to the district health boards, back then every district health board was cost under pressure, so where do you go when you're not having to save on doctors and nurses, you go and see what you can save in the IT and your trying to make your old equipment tick over," he said.