Whanganui District Health Board owes its staff more than $4.8 million in leave, with one in three employees owed more than four weeks.
Official Information Act documents released to the Labour Party show unpaid leave balances around the country are high.
Seven health boards owe at least 40 per cent of their staff more than their annual leave.
Labour's health spokeswoman Annette King said high leave balances showed the Government was underfunding the healthcare system.
"We're now seeing the results - health professionals and other health board staff so overworked they have had to put off taking millions of hours of annual leave," she said.
"Such large numbers of missed holidays show just how stretched our health sector has become. Thousands of health professionals are putting off a break because there is no one to cover when they are away."
Whanganui health board acting chief executive Brian Walden said the leave balance was representative of an organisation with 1000 employees.
He said doctors and shift workers received five or six weeks of annual leave, so one-third of staff accruing more than four weeks "is not a surprise". He added that the health board encouraged staff to take leave.
Nationally, health boards plan to save $163.5 million this year, with Whanganui hoping to save $2.66 million - $350,000 of that coming from more efficient nurse use.
Mr Walden said this would manage "the peaks and troughs of service flows more effectively".
Ms King said the move to compel savings from health boards came after years of pressure from central government, while Association of Salaried Medical Specialists executive director Ian Powell said over-stretched staff being placed under increased pressure put patient safety at risk.
He said a recent survey by the association had revealed sick staff coming to work when they should be at home, including one doctor who was being treated in the emergency department between seeing patients.
Public Service Association national secretary Erin Polaczuk said staff were not taking leave and some were working double shifts to cover roster gaps.
Across the country, health boards owe more than $465 million in leave to almost 70,000 employees.
However, Health Minister Dr Jonathan Coleman said the health budget was at a record high of $15.9 billion which was $400 million more than last year.
There had been no funding "cuts", he said, and the Government's "careful management of the health budget" had reduced health board deficits by $150 million since 2008.