Air New Zealand grounded its entire fleet of ATR 72-500 turboprops on 18 March after discovering hairline cracks around the cockpit windows of one aircraft.
Two of the carrier's 11 ATR 72-500s were quickly inspected and returned to service within hours, ANZ says.
Three aircraft require closer examination, five are "well-advanced" in the inspection process and one aircraft was in already grounded for planned maintenance, the Star Alliance carrier says.
The 68-seat ATR 72-500s, which are operated by Mount Cook Airline, are 10.9 years old on average.
Starting on Monday, ANZ will supplement Mount Cook Airline with some aircraft from the mainline fleet to restore two-thirds of the regional carrier's seating capacity.
The cracks were discovered during routine maintenance a Mount Cook Airline's base at Christchurch Airport.
Two years ago, ANZ moved heavy maintenance work for Mount Cook Airline's ATR 72 fleet from Christchurch to Nelson, the home of sister regional carrier and Bombardier Q300 operator Air Nelson.