Cracks have been found on eight of Air New Zealand's fleet of 11 ATR 72-500 turboprops.
"These cracks can appear after a high number of cycles and can be detected during a standard maintenance check scheduled at 24,000 cycles - about 10 years of operations," says an ATR spokeswoman.
She added that the cracks were discovered in the "frame 5 segment" during a cockpit window change, but reiterated that the cracks do not compromise aircraft safety.
Three of the carrier's ATR 72-500s will have to undergo a technical adaptation while another three will require a frame-segment change.
On 20 March, Mount Cook Airlines, Air New Zealand's regional subsidiary, is able to operate four of the aircraft.
Three more turboprops are expected to be operational by 21 March and the remaining by the end of the week.
The carrier first grounded its entire fleet of ATR 72-500s when hairline cracks were discovered around the cockpit windows of one aircraft during routine maintenance.
"There is no safety issue with these cracks, as [the] mechanical capability of [the] cracked frame versus operational flight and ground loads was demonstrated," says ATR.
The airframer added that "it is not necessary" for ATR operators to push forward their maintenance check as the cracks do not compromise safety.
"ATR recommends proceeding through nominal maintenance planning and relevant repairs in case of findings [sic]," says the spokeswoman.