"But it made no noise as it flew past and, when it landed on a branch, I saw it was definitely a morepork."
As well as its size and shape, the bird had the distinctive morepork eyes and flight.
Mr Robertson said he barely had time to empty his pack and find his camera before it flew off.
Since he posted the photo online some people have suggested it is a barn owl, an Australian species of which a few are known to live in the Far North.
But barn owls, although white, are much larger, have different faces and live in open ground or at forest edges.
Trust chairman Gerry Brackenbury said the trust's kaumatua, Richard Shepherd, had declared the sighting a good omen as well as a sign of the forest's uniqueness.
White ruru featured rarely in Maori mythology, and were considered good luck rather than an unwelcome omen. One seen at the outset of a task was taken as a sign the goals would be reached. If a hunting party saw one as they set off, the hunt would be successful - if a war party saw one, the fight would be won.
Museum of New Zealand Te Papa vertebrate curator Alan Tennyson said this was the first time he had heard of one being seen in the wild, although Walter Brook wrote in the 1955 edition of his "bird bible", New Zealand Birds, that there had been reports of albino morepork.
Albinism occurs in all animal species and is usually accompanied by poor vision. Mr Tennyson thought the "extremely uncommon" Pukenui find was more likely to be leucism, a condition which makes birds' feathers pale or white.
He agreed with Mr Robertson and Mr Brackenbury who thought a morepork would not survive if it could not see properly, as they relied on eyesight to swoop to catch flying insects or small ground creatures.
-See the Pukenui Forest feature story in the 48 Hours section