Aircraft have flown too close to one another over Auckland 19 times in two years.
The Airways Corporation, which runs air traffic control, disclosed the figure after two incidents in one day, when airliners about to land at Auckland Airport were instructed to pull up and try again because the runway was obstructed by departing planes.
Despite dramatic-sounding accounts from witnesses on the ground, Airways insisted there was no loss of required separation in either case, and there was "nothing unsafe or untoward in go-arounds being requested or directed".
But in response to a subsequent Official Information Act request, it disclosed 19 "loss-of-separation" incidents above Auckland in the two years to August. Away from the airport, planes must stay three nautical miles apart laterally. The default vertical separation distance is 1,000 feet.
Last night, corporation spokeswoman Philippa Sellens described the breaches as "technical" and said no near-collisions at or around Auckland Airport were recorded in its database.
But she refused to provide details of the incidents - or what type of aircraft were involved.
Airways' corporate solicitor, Kylie Campbell, wrote earlier that it was obliged to protect information from industry sources which was needed to promote aviation safety, but said the Herald was entitled to apply to the Ombudsman for a review of the corporation's decision.
She also disclosed air traffic controllers had instructed pilots 68 times in the past two years to postpone landings and "go around" Auckland airspace before trying again.
"The majority of go-arounds occur due to sudden changes in weather which is, of course, characteristic of the challenging operating environment in New Zealand," Ms Campbell said.
But that was not so with the two "go-arounds" reported on August 4, which occurred after passengers on the ground had faced take-off delays.
Ms Sellens said then they were not weather-related, but offered no other explanation.
An Airline Pilots Association spokeswoman said her group was unaware of any lost separation in the past two years, and would need more details to make a serious comment.
She advised against making the public "anxious when there is no need to" and suggested Aucklanders should be more nervous about being tail-gated in their cars.
Robin Scott of Mt Eden, a member of the Plane Truth campaign against new flight paths over Auckland homes, said the consequences of a two-car crash were "nothing compared to aircraft having a mishap".
"We can see the ribs on the aeroplane as it's going over. It would be a concern if they got a bit close - how are they going to get out of each other's way?"
A New Zealand-based editor for the international Aviation Week publication, Adrian Schofield, was surprised by Airways' numbers - because they sounded "quite low".
"It would be important to not be alarmist about it because in any airport system in the world you're going to have loss of separation," he said.
"They have these pretty conservative separations in terminal airspace and it doesn't take much to violate the separation."
Such incidents in the US, where he spent about 10 years at Aviation Week's head office, "can run into the thousands nationwide and into the hundreds at individual airports".
In her response to the Herald's Official Information Act request, Ms Campbell said aircraft operating over Auckland City, but outside the vicinity of the airport and subject to radar control, were required to keep three nautical miles (5.6 km) apart laterally.
Required vertical separations varied according to flight tracks, but if no height was prescribed, the minimum distance was 1,000 feet (305 metres).
Separation distances in the vicinity of the airport were determined by air traffic controllers based on factors such as aircraft type, weather conditions and whether instrument or visual flight rules applied.
The Civil Aviation Authority said it was unable to respond to inquiries on the issue until today.
Aircraft "go-arounds" above Auckland Airport
• August 2012 - July 2013: 37
• August 2013 - August 2014: 31
Loss-of-separation incidents between aircraft above Auckland and the airport
• August 2012 - July 2013: 13
• August 2013 - August 2014: 6
- Source: Airways Corporation