Department of Defence documents previously marked "SECRET" show the Royal Australian Air Force moved to stop investigating UFO reports in the 1990s because the majority weren't visitors from outer space and only about 3 per cent of sightings could not be explained.

The policy statements and correspondence, released through freedom of information laws, outline a new policy for RAAF personnel to refer reports of sightings to civilian UFO groups.

Between 1960 and 1973, about 815 UFO sightings were reported to Australian authorities.

About 90 per cent were later attributed to aircraft, satellites, meteors, space debris, stars and planets, while seven per cent could not be investigated due to timing or lack of information.

The truth will remain out there for the remaining three per cent, attributed only to "unknown causes".

The documents show officials chose not to announce the change to the media to avoid conspiracy theories and controversy.

One official said any "extra-terrestrial threat to Australian security [was] not likely to develop without some foreknowledge from astronomical and other surveillance systems".

"The potential exists for the RAAF to be accused of withholding documents about particular sightings or incidents, or neglecting our national security obligations by not taking these matters seriously," one said.

Members of the public reporting sightings which could have national security implications, including "man-made debris falling from space or a burning aircraft", would be instructed to contact police or civil aviation authorities.

Hundreds of reports of "flying saucers and other aerial objects" held by the National Archives of Australia include sightings of large discs and rockets in the sky, bright lights, kaleidoscopic colours, explosions and even a domed craft with lines of lights eminating to the ground.

UFO Research NSW spokesman Doug Moffett said governments used a "round peg, square hole" approach for thousands of sightings every year.

"Whether you're taking about the air force or the government, it is just a no-go area," he said.

"Government departments think there is nothing to be gained by becoming involved with it.

"Western, English-speaking governments pretty much all sing to the same tune. America's best buddies - us, England and Canada - all treat the UFO thing pretty much the same, as a non-subject."

Mr Moffett said he didn't believe sinister motives drove the change in policy but governments had long sought to diminish the reality of UFOs.

The original 1984 policy was reviewed after at least two investigations of unusual sightings, including one when strange lights were seen in the sky around the Victorian city of Bendigo in 1983 and one when Mirage and F-111 aircraft were brought to "high states of alert" because of an object spotted at low levels on the Sydney radar.

The Bendigo investigation was inconclusive and the Sydney radar was later found to be faulty.

Conceding some sightings in Australia and overseas could never be explained, the documents include a questionnaire once used to help record information.

Witnesses were asked to specify locations, weather conditions, the direction the object was travelling in and its angle to the horizon.

Reports could include a description or sketch of the object, including its colour, size, shape, brightness, speed, sound and propulsion as well as physical evidence like indentations and scorching of the ground.