Urgent government action is needed to stop doctors burning out and risking patients' live, say medical specialists.
The Association of Salaried Medical Specialists said doctors at public hospitals were reporting unrelenting workloads, fatigue and severe burnout.
Patients could suffer as a result of overwhelming working conditions, the union's executive director, Ian Powell, said.
Understaffing and onerous on-call duties are also contributing to what he is calling a "burnout crisis" - with an estimated 50 percent of specialists experiencing the problem.
"There is a significant increase in numbers of acute patients coming into public hospitals at a rate significantly greater than staff growth," he said.
Doctors were confronted with serious staff shortages, excessive hours, increasing workloads and many colleagues leaving, he said.
Mr Powell said the government had inherited the problem from the previous government's lack of funding over eight years.
He is urging Health Minister David Clark to require district health boards to prioritise doctors' well-being.
"The responsibility is really the district health boards' that run our hospitals, but there has been a significant dereliction of duty from them in recent times," Mr Powell said.
Health boards had known about specialist burnout for a number of years and failed to acknowledge the severity of the problem, he said.
It would be more financially prudent to hire permanent staff over expensive temporary consultants, who were used to fill the gaps, he said.
"That also increases the risk of medical error, [which] becomes more costly."
Health boards have the money to encourage an extra 800 to 1000 people into the workforce, Mr Powell said.