The Airways Corporation is investigating whether lightning strikes may have caused the failure of runway lights at Auckland International Airport.
Twenty-two flights in and out of the city had to be diverted or delayed after the lights failed about 8.10pm on Saturday because of a cable fault.
About 2000 people had their travel plans disrupted, with hundreds spending an uncomfortable night camping at Auckland, Wellington and Christchurch airports.
After exhaustive testing, airport operations were fully restored at 5.30am on Sunday.
The Airways Corporation is responsible for the lights.
Its operation manager, Lewis Jenkins, says lightning strikes are being investigated as a possible cause because there was a similar event in 2008 which was found to be related to lightning activity around the airport.
He says around a dozen staff are investigating the fault.
At least 17 flights had to be grounded or diverted to Wellington, Hamilton or Christchurch. Two planes from Vanuatu were turned back, and a flight from Chile was diverted to Tahiti.
Air New Zealand said on Sunday evening that it still had passengers delayed at Los Angeles International Airport as a result of the lights failure.
Air New Zealand said the inconvenience to passengers is unfortunate but the delays were outside the airline's control and it is not required to offer compensation.
Forced to spend night at airport
Passengers stranded at Auckland and Christchurch were forced to camp out in the terminals overnight.
Among those in Christchurch were a group of Tongan students on a flight from Nuku'alofa, who speak little English and had no idea where they had landed.
The chairman of the Tongan Advisory Council, Melino Maka, says he kept an eye on the boys but it was a stressful introduction for them to New Zealand and their parents in Tonga had been frantic about their whereabouts.
An Air New Zealand passenger, Toni Kosterman, whose flight from Tonga ended up in Christchurch, says there was chaos at the terminal.
An Auckland couple also returning from Tonga, where they celebrated their 25th wedding anniversary, say they were told just as they were about to land in Auckland that they were being diverted to Christchurch.
Wendy Bowen says she and her husband, Bryce Bowen, were promised hotel accommodation but ended up sleeping overnight at the terminal.