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.

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Gio said it was likely to be flying at 32,000 feet (9.7km) and not straight up into the air, despite numerous reports from people who saw it.

The reason it appears to have been flying so steeply was because of the curvature of the earth he said.

The vapour trail from Auckland. Photo / Scott Moreton
The vapour trail from Auckland. Photo / Scott Moreton

New Zealanders throughout the North Island reported spotting the mysterious vapour trail, or contrail, stretch over the sky this afternoon.

Simon de Prinse said he saw the C-17 flying only about 100m above Uretiti Beach south of Whangarei earlier today.

"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."

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Aircraft sometimes leave white trails, or contrails, behind them due to the humid exhaust from jet engines mixing within the atmosphere.

The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse
The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse

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.

The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse
The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse

"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."

Others who spotted the mysterious aircraft searched flight radars online but were unable to find any conclusion as to which flight caused it.

The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse
The C-17 aircraft flying above Uretiti Beach south of Whangarei. Photo / Sion de Prinse

A man who spotted it from Takapuna in Auckland said he couldn't find a call sign for the aircraft and said it was "honking" along.

A Herald reader from Raglan said the aircraft seemed to be flying on a steep ascent, while a man from Hamilton said it was flying low over the city.

"Looking at the height of it, it appeared too high to be landing at Hamilton Airport but very low for a jet," he said.

The trail appeared to have started near Whenuapai Air Base in northwest Auckland. Photo / Zachary Adamson
The trail appeared to have started near Whenuapai Air Base in northwest Auckland. Photo / Zachary Adamson

"The plane few southwest over Hamilton early this afternoon. Clearly visible and not particularly high. Very wide trail.

"I have seen them in Europe on numerous occasions but nothing this wide and low. The whole staff were looking at it out of the window."

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.

The aircraft flew over the horizon within minutes. Photo / Supplied
The aircraft flew over the horizon within minutes. Photo / Supplied

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.