Clinical services across all Waikato public hospitals have been seriously affected by a cyber security incident with all phones and computers down.
Outpatient appointments and patients coming for a non-urgent surgery have been affected.
Resident Doctors Association and Association of Professional and Executive Employees (APEX) national secretary Dr Deborah Powell says the cyberattack on the Waikato District Health Board appears to be a type of ransomware called "Conti".
It appears to be same type of attack that targeted Ireland's Department of Health last week.
"Some elective surgery has [been cancelled], some have still gone ahead, but some surgery has been cancelled," Waikato DHB chief executive Kevin Snee told Checkpoint.
"We're talking about a number of patients, but at this stage we can't be clear exactly how many because we're in the middle of it … it will only be clear about how many patients have been affected after the event.
"It's a cyber security breach. It's a virus that's affected our system and it looks like that's been malicious intent."
Snee said it could "well be" a ransomware attack.
"Certainly, the indications are that it is.
"We've received a message - we need to check the veracity of the message - but it appears to be indicating that it is a ransomware event.
"Obviously, when these events happen as they happen in other parts of the world, people are trying to extract money from an organisation. Clearly we will not be paying any ransom for this."
He said Waikato DHB received the message earlier today, and the attackers had not asked for a specific amount of money.
"We've had no threats [in the past]. It's a problem that we're aware of internationally and we've had problems in New Zealand before, but we've had no threats in relation to Waikato DHB.
"We had some problems this morning with telemetry, however, that's been dealt with. The risk has been mitigated there and we do have problems, for example, with our ability to read X-rays and lab tests. Submissions are having to go down into the Radiology Department to read X-rays.
"It's the whole computer system that we have a problem with at the moment.
"If there have been any security issues, that will be determined by investigation subsequently.
"Clearly, it's a security issue if you've been hacked. The question is how could we have prevented that? And any investigation subsequently will tell us that."
Snee said the DHB was working with cyber security experts from the Ministry of Health to find the source of the attack and rebuild the computer system.
He said they had made a number of improvements over the past year, and had not been told there was a problem.
"We hope to have this rectified before the weekend, but exactly how long it's going to take, at this stage, we can't be certain.
"We're working with both our regional and national partners to make sure that any patients who need urgent surgery, get it.
"Either get it locally with us or in our regional partners or with our national tertiary partners. So anybody who needs urgent surgery should get that.
"Many will be done in Waikato itself, but then it may be necessary for some other patients to be diverted to other hospitals.
"Elective surgery for cancer patients will go ahead, but if you don't have cancer, then you may not receive elective surgery. We'll contact patients directly to let them know whether they're going to have their surgery or not."
Snee would not give details on what the attackers were demanding or if any further threats were made.
"It's a police matter, so I'd rather not go into it.
"We believe we've got it under control and we believe we'll have the services back up and running within a matter of days.
"We don't believe that any further problems will occur as a result of this… We're focused on patient care and getting the systems back up and running. That's what we intend to do.
"Irrespective of any further threats that may or may not happen, we will have the system back up and running."
The National Cyber Security Centre has confirmed it is providing support for the Waikato DHB.
The Cyber Security Centre works within the Government Communications Security Bureau.