Staff at Wellington Hospital have questioned the DHB's assertion that a repair job on the building's pipes would not affect patients.
The water pipes in Wellington's seven-year-old hospital are riddled with tiny holes and will have to be ripped out and replaced.
The health board has confirmed pinhole leaks have been discovered in the regional hospital that cost $346 million to build.
The leaks are extensive in both the hot and cold water pipes across the more than 48,0000-square metre hospital in Newtown.
Capital and Coast District Health Board has told staff the leaks will cause disruption for staff but not for patients.
Georgia Choveaux from the Nurses Organisation said while it would possible to manage the disruption, the DHB was facing a $4m deficit.
"Where's the money coming from and, if it's not going to be met by a capital injection from the government - or the contractors themselves meet the bill, then it probably will have an impact on patients and staff in that the health sector is already underfunded," she said.
"So cuts would need to be made to meet these bills."
Capital and Coast District Health Board would not comment when asked what the repair bill might amount to.
The board's chief executive Debbie Chin last night said they were still investigating the root cause and considering what action to take.
Fletcher Construction was the main contractor for the hospital build that ran from 2005 till 2008, with the opening a year later.
Fletcher told RNZ News it had not been contacted by the DHB since the copper plumbing got its practical completion certification years ago.
It had a good relationship with Capital and Coast and was doing some other work at the hospital at the moment, it said.
Aquaheat Industries put in the pipes. It is now a struck off company, but back then was owned by Australian engineering conglomerate Hasties.
Hasties collapsed, and Whakatane's Horizon Energy bought the assets from receivers in 2012 and now runs the company as Aquaheat New Zealand.
Horizon's chief executive Ajay Anand said Aquaheat NZ was investigating what responsibility if any it had for work done by Aquaheat Industries.
The company was willing to work with Wellington Hospital and head contractors to understand the problem then settle on a fix if any was needed, he said.
Wellington City Council, which signed off all the work under the building code, said the hospital leaks were "not on its radar".
It is understood hydraulic engineers have been reporting back to the DHB about the leaks for over a year.
RNZ News has asked to be given those reports.
A reticulated system such as the hospital's runs 24 hours a day, with the water going round and round.
RNZ News understands it might not just be the pipes but also the impellers that help move the water around that are damaged.
Copper can be pinholed by chlorine attacking impurities in the metal. Chlorine becomes more corrosive as the water temperature goes above 65 degrees Celsius.