A case in which two commercial Boeing 737 planes were too close to one another while landing at Queenstown International Airport is being investigated by the Transport Accident Investigation Commission.
Investigator Ken Mathews said the commission was investigating after the incident at 10.40am on Sunday.
He said the Pacific Blue and Qantas planes were approaching the airport when a possible "loss of separation" occurred.
"It wasn't close. They were about four miles (6.4km) apart," he said.
A report was unlikely to be completed before April next year.
Civil Aviation Authority spokesman Bill Sommer said Queenstown's air traffic control unit, Airways Corporation, had notified the authority about the incident.
The authority had passed the investigation over to the commission, he said.
Airways Corporation spokeswoman Nikki Hawkey said it would be inappropriate to comment while the investigation was under way.
It is the second incident involving passenger plans flying too close near Queenstown this year. he Civil Aviation Authority had investigated an incident over Lake Wakatipu during which two passenger planes came close enough to spark an automatic warning to the two pilots.
Authority spokeswoman Emma Peel said at the time an Air New Zealand Boeing 737 was approaching Queenstown Airport's runway on January 16, when it was warned by its "terrain collision-avoidance system" another aircraft was in the area.
