Eagle Airways says it's confident the airline's Beech aircraft are safe after 14 of them were grounded on Tuesday when a hairline crack was found in the tail of one of them.
Eagle, an Air New Zealand subsidiary, operates 18 Beech 1900D aircraft out of 20 domestic airports.
Its general manager, Carrie Hurihanganui, told Morning Report cracking like this is common.
"It's not unsual - an aircraft which is metal - you do find that through the inspections and the regular maintenance, and it is about dealing with them before they become a significant issue.
"This aircraft is 10 years old, which is not old in aircraft standards, but it does require maintenance over time."
Engineers were doing checks during maintenance of one plane on Tuesday when a small tail crack was found. The rest of the fleet was then checked.
Air New Zealand says only four of the aircraft will fly on Wednesday.
It will not confirm if all 14 of the grounded planes have the same problem, saying only that it is consulting with the manufacturer.
The airline says about 1000 passengers could be affected, and its other planes will fill the gaps in the meantime.
Aviation commentator Peter Clark says aircraft fatigue does happen and small cracks or fractures will occur, but the important thing is to nip it in the bud before it becomes a major problem, so Air New Zealand has followed the right procedure by checking the entire fleet.