Thirty-six per cent of staff are owed a total of $3 million in annual leave payments, with the amount growing by almost one-fifth over the last five years.
The demand for nationwide savings has prompted claims of increased risks to patients due to stressed out and tired staff.
Labour health spokeswoman Annette King said the move to compel savings from health boards came after years of central government pressure.
"They've squeezed this lemon so hard the efficiency gains are going to be so difficult to find."
She said health boards targeted personnel costs because filling or not filling vacancies was an area they could control, unlike external costs.
Public Service Association national secretary Erin Polaczuk said hospital staff around the country were not taking leave and some were working double shifts to cover roster gaps.
Association of Salaried Medical Specialists director Ian Powell said patient safety would be jeopardised by over-stretched staff being placed under increasing pressure.
He said a recent survey by the association had revealed that sick staff were coming to work when they should be at home, including one doctor who was being treated in the emergency department between seeing her own patients.
Mrs Isbister was confident the planned savings could be made without affecting frontline services. She said the board had made savings elsewhere, such as $67,000 through price negotiations for hospital supplies and more than $40,000 after renegotiating vehicle leases.
Nationwide, the Ministry of Health expected DHBs to save $163.5 million this year, with most health boards around the country aiming for multimillion-dollar savings. Four DHBs are hoping to reduce net spending by more than $10 million each, with Waikato DHB aiming for savings of $43 million -- 4.4 per cent of its almost billion-dollar budget.
Health Minister Dr Jonathan Coleman said the health budget was at a record high of $15.9 billion. He said it was $400 million more than last year and there had been no funding "cuts". The Government's "careful management of the health budget" had reduced health board deficits by $150 million since 2008.