Labour health spokeswoman Annette King says pressure on district health board staff is due to insufficient funding from the Government.

Health workers across New Zealand are owed more than eight million hours in annual leave, with fears overworked staff could make dangerous errors if they cannot take time off.

Information obtained by Labour showed staff at 18 district health boards (DHBs) were owed 8.6 million hours in annual leave as of June 2015 - an average of more than two weeks of leave per staff member.

Labour health spokeswoman Annette King said the "shocking" figures backed up reports of extra pressures on staff who were unable to take leave.

"Now that's bad for the health system if you put that much pressure on staff, because like the rest of us...everybody must have a break for their own health."

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King said the pressure on DHB staff was due to a lack of adequate funding from the Government, with data commissioned by Labour last July revealing that health funding had been slashed by $1.7 billion in the last five years.

There was a risk of medical staff making more errors if they did not get the time off they needed, she said.

"When staff are under huge pressure and they can't take leave, particularly if someone is away sick and there's a gap and they have to work longer hours...then you're putting together a recipe for a real problem."

The Public Service Association, which has 16,000 members working for DHBs, said the figures backed up what it had heard.

PSA national secretary Erin Polaczuk said Labour's claims "resonated" with her, as the union had been told about high levels of stress and bullying in the workforce.

"I tend to think there's a link between bullying and being overworked, that sort of high-stress environment."

Polaczuk said fatigue presented a serious risk to the "incredibly important" work that health professionals did.

"We could end up seeing our members becoming sick, or mistakes happening that shouldn't have happened because people haven't had enough time to rest and recuperate."

The Government needed to address a "fundamental problem with underfunding" in health, she said.

A spokeswoman for Health Minister Jonathan Coleman declined to comment, saying it was an "operational matter" for the DHBs.

Coleman has previously defended government funding of health, saying the sector was well-funded, though more work was required to target funding where it was needed.