A mystery object seen in skies over the Tasman Sea near Kaitaia two months ago was most likely a commercial aircraft.

That is the conclusion Wekaweka amateur astronomer Bill Keir has drawn after investigating photos taken by a man at Ahipara on April 28 at 5.42pm.

Mr Keir, who studied three Northland UFO sightings in the 1980s, visited Ahipara last month to determine the location of the object using scientific measurements.

The object in the photos could not have been an astronomical body, such as Venus, he says.

Information provided by Airways Corporation of New Zealand points to it being the contrails of an aircraft bound for Australia.

Emirates flight EK433 - which leaves Auckland for Brisbane each day at 5:12pm - would have been about 75 nautical miles west of Kaitaia at 34,000 feet when the photo was taken, he says.

At this altitude and distance, the aircraft would not have been easily visible from Ahipara. Any vapour trails would have been lit up by the sun.

UFO Focus New Zealand has classified the sighting as an unusual aerial phenomenon - UAP - saying there were no scheduled flights in the area at the time.