A monster military transport aircraft from the Australian Air Force is thought to be the source of a mysterious vapour trail which stretched across the North Island.
The Herald understands a Royal Australian Air Force C-17 aircraft, which measures to 53m long and a wingspan of nearly 52m, caused the vapour trail.
The aircraft is said to have been transporting Royal New Zealand Air Force NH90 helicopters back from the Solomon Islands.
Kieran Gio from Radar5 said the Australian aircraft was being used because the helicopters are too big to fit in any of the RNZAF's current fleet.
"It would have been no more the 100 metres up in the air. Photos were taken with an entry-level cellphone to give you an idea of how low it was," de Prinse said.
One woman who spotted the vapour from Highbrook, East Auckland, said the aircraft was travelling too fast to be an ordinary passenger aircraft.
"It had gone more or less straight upward and I started seeing the curve as shown in the video after another three minutes," she told the Herald.
"The video was taken from Highbrook Business Park on Lady Fisher Place and the plane had disappeared over horizon within five minutes of first noticing it."
Aircraft sometimes leave white trails, or contrails, behind them due to the humid exhaust from jet engines mixing within the atmosphere.
They are primarily composed of water in the form of ice crystals, spreading over the sky as aircraft fly to their destination.
Footage of the contrail in Auckland shows the aircraft which caused it appeared to fly high into the air before curving off and heading into the distance.
Meanwhile, another reader said he saw the strange vapour over Auckland city leading southwards about 2.15pm.
"I work in an office building in downtown Auckland and I noticed it from our lunchroom," he said.
"It appeared to be flying high above the North Shore and heading southward but also still climbing high in the process. It was definitely flying very high at the time."
A man who contacted the Herald said the aircraft had twin engines on both wings and was heading eastwards as he looked at it from Riverhead.
Meanwhile, a reader spotted the aircraft from Cockle Bay, East Auckland, and said the aircraft was flying south or southeast.
Another man said he pulled over his car at Waiuku after spotting the contrail and checked the flight radar app but was unable to find the mysterious source.
The Herald has asked the Defence Force and the Civil Aviation Authority for comment.