Engineers have found that large parts of Taranaki Base Hospital - including its emergency department, intensive care unit and maternity block - are probably earthquake prone.
That is the weakest seismic rating, where part of a structure is less than 34 percent as strong as a new building of the same type on the same site would be.
The ranking covers four buildings that provide medical services - including radiology, renal, and speech language therapy - three other buildings including offices and the mortuary, and three tunnels that supply vital services.
The district health board is waiting to confirm the assessments but wanted to give patients and staff an early heads-up, said DHB chief executive Rosemary Clements.
"We're monitoring and measuring what's going on at all times," she said.
"We're putting together a seismic planning report on what it is that we'll need to do over the next few weeks, the next few months."
There were no critically weak parts that needed an immediate fix, she said.
Consultants were now looking at what risk-mitigation work might be needed.
It was not possible to relocate acute medical services for the one or two years needed to strengthen the existing buildings, so instead the aim was to construct new buildings by 2023 "or sooner", Mrs Clements said.
The DHB has just got the nod for the next phase of plans for new buildings from the Health Ministry, which are estimated to cost $270 million.
Hospitals nationwide have at least 110 earthquake-prone buildings; 15 of these are of the highest importance level, which includes clinical blocks but also the likes of unoccupied boiler rooms that supply vital hot water, or IT units.
Taranaki rates as medium risk among the country's three quake-risk zones.
None of its hospital buildings were rated as earthquake-prone in previous assessments, but have come up short under tougher engineering assessment rules introduced in 2016.
Hawera Hospital has yet to be assessed.