From 14 July, doctors, nurses and other health workers will no longer be able to claim special paid sick leave for Covid.
The Public Service Association (PSA), which represents 25,000 health workers, has told its members to continue to stay home if sick, and contact the union for support if they felt pressured by their employer.
PSA national health sector lead Andrew Skelly said people working in healthcare must be able to stay home when they get sick, or people's lives would be put at risk.
"This is simply irresponsible at a time when we are seeing a surge in Covid cases - taking away sick leave will cause more Covid transmission in hospitals amongst ill and vulnerable people."
The country was just emerging from its sixth big wave of Covid transmission in 30 months, which had seen 55 people a day hospitalised at its peak, he said.
The PSA is among a number of unions urging Health New Zealand to reverse its decision.
"We are seriously concerned about long-Covid and other significant health impacts on people working in every part of our hospitals to keep them running, administrators, anaesthetic technicians, social workers, everyone.
"Health workers are more likely to get multiple Covid reinfections by virtue of where they work. Their employer should be doing everything possible to protect them and patients."
While Te Whatu Ora had advised managers to grant "discretionary leave" to workers who run out of sick leave, that had always been difficult to access or only paid after someone returned to work.
"We have been given no rationale. This appears to be another penny-pinching mechanism chipping away at public health at the expense of patients and workers," Skelly said.
Health New Zealand has been approached for comment.
