The 12-bed facility - stage one of a two-phase redevelopment of the former intensive care and high dependency departments - was due to begin being used by patients and staff next week.
However, last-minute checks had revealed a problem with the unit’s isolation room, Southern District Health Board chief executive Chris Fleming said in a letter to staff.
"Our expectation was that the unit would be commissioned from next week. In preparation for this, additional testing has been taking place, which has returned unsatisfactory results relating to air pressure in the isolation room," he said.
"These results were a surprise to us, and we need to take time to understand them and determine our options for addressing this."
The SDHB had decided to delay using the new unit in the interests of the safety of patients, staff and families, Mr Fleming said.
No time was given for how long that might be.
"We are not yet sure how long the commissioning of the unit may be delayed for, and are hopeful it can be addressed promptly," he said.
"We share the disappointment of the ICU team regarding this delay, and acknowledge the frustration and inconvenience this creates for other teams, and those you are caring for."
Staff at the ICU have anticipated the move to new quarters for years. The current unit is dark, cramped, noisy and affords little privacy to patients or families.
The new unit has natural natural light in most areas, private rooms, dimmable bulbs and acoustic ceiling tiles to dampen noise.
Each bed space has new monitoring technology, and the central nurses bay includes sight lines and monitors which allow staff to monitor patients in all rooms.
More space had also been made available for families, staff and students.
Stage 2 of the redevelopment, which has an extra 10 bed spaces, is expected to open mid-2019.
