Doctors at Waikato Hospital want Te Whatu Ora to address severe staff shortages, after major gaps at the hospital over the weekend.
The number of medical registrars was down to 40 percent of normal on Saturday and Sunday.
Te Whatu Ora said recruitment was well underway.
Resident Doctors Association national secretary Dr Deborah Powell said numbers were extremely low.
"This 3-month period we have half the number of medical registrars we need to staff the hospital, but things are getting worse with this last weekend down to 40 percent and next weekend looking equally if not more bleak."
Powell said the doctors were burning out.
"They are also susceptible to winter illnesses exacerbating an already dire situation. Waikato has no answer to the situation other then to reduce the care patients would normally get from their doctors."
One area service that had been reduced was for surgical patients who in addition to their surgical condition had a medical problem.
"It is normal for the medical registrars to assist medical management of surgical patients...but this service has had to be withdrawn simply because we don't have enough doctors to do the work," she said.
The union said it had been working with Te Whatu Ora on a raft of initiatives to address the loss of doctors from the public health system.
However, Powell said the work had all but stopped whilst management redirected their focus to budgets.
"For all the rhetoric about strengthening front line care, the fact is that both patient care and the workforce we need to provide that care are suffering and the situation is getting worse.
"We have options, we can strengthen our workforces," Powell said. "But Te Whatu Ora needs to support us to do so which currently is not happening."
Unplanned leave and vacancy
In a statement Te Whatu Ora said the shortage of medical registrars this weekend was caused by unplanned leave and a vacancy.
They said senior medical officers and locums gave support to ensure services continued.
"Patient safety is always a priority and when the roster gaps were identified additional staff were rostered on to ensure critical patient services were not impacted.
"Rostering in a dynamic hospital environment is always a challenge and the co-operation and flexibility of our staff in managing these situations is appreciated."
It said recruitment of medical registrars for the hospital was ongoing with a number of candidates accepting offers.
Waikato Hospital should be fully staffed with medical registrars by the beginning of October, Te Whatu Ora said.