The Civil Aviation Authority (CAA) says it will investigate what caused a radar fault that affected nearly 200 flights this afternoon.
Airways, which monitors New Zealand's airspace, said it noticed a fault at about 2.40pm.
The fault meant that planes already in the air could land, but those yet to depart were grounded.
The radar went back into full service at about 4.30pm but delays to flights continued tonight.
The CAA said the fault was caused by an internal system network error at Airways.
It said its investigation would also focus on trying to prevent the fault from happening again.
It said the scope of the investigation would depend on whether the Transport Accident Investigation Commission also decided to review the incident.
A spokesperson for the Airline Pilots' Association, Lisa Williams, said air traffic controllers had to revert to emergency procedures to allow about 50 planes already in the air to land.
She said controllers had to rely on communicating directly with pilots to work out the position of planes.
"You're right to say that air traffic controllers do need to be praised for this - because, look, it was a stressful situation."
Flight delays expected
Air New Zealand had 160 flights affected during the fault while Jetstar had six flights delayed.
Auckland Airport spokesperson Simon Lambourne said this afternoon that five international jets and about 14 domestic flights were delayed.
He said the airport had been prioritising departing aircraft.
"We're departing aircraft at a rate of about one every five minutes, so we're just working through in order of priority - firstly the international jets, then the domestic jets, then the regional aircraft."
At Wellington Airport, Air New Zealand flights to Napier and New Plymouth were cancelled.
Most services at the airport have resumed but some have been delayed by up to an hour.
'Internal network failure'
Airways also said the cause of the fault was an internal network failure. It did not go into detail this afternoon about what that meant but said no passengers were at risk.
It apologised for the inconvenience caused by the radar problem.
Airways, which is a state-owned enterprise, controls air traffic throughout New Zealand. It has two main radar centres, in Auckland and in Christchurch, and employs about 750 staff.
Transport Minister Simon Bridges said air traffic control was still able to communicate with incoming planes via radio contact after the fault.
He said he had been assured there was absolutely no compromise to the safety of passengers.
Operations have been resumed, and expect to resume full service by 16.30. We apologise for the inconvenience caused.— Airways New Zealand (@AirwaysNZ) June 23, 2015
Pilots saying we are cleared for takeoff very soon. Everyone back to seat! Yay! #GreatPlaneGroundingof1815 thanks Airways NZ— Paul Le Comte (@five15design) June 23, 2015
False start... ChCh Civil Aviation radar is buggered. Sitting on Tarmac...— Paul Le Comte (@five15design) June 23, 2015