Although federal and state government agencies refused to comment about the 1966 ‘Westall’ incident at the time, it is now believed that, rather than a UFO, what landed was an errant high altitude balloon used to monitor radiation levels after the controversial Maralinga nuclear tests.

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The HIBAL program was a joint US-Australian initiative to monitor atmospheric radiation levels using large silver balloons equipped with sensors between 1960 and 1969.

Documents held by the National Archives and former Department of Supply indicate one test balloon launched from Mildura may have been blown off course and came down in Clayton South in a paddock near Westall High School, alarming and baffling hundreds of eyewitnesses, including teachers and students.

After hovering over the area, it landed at an area known as The Grange, behind a grove of pine trees, before taking off again and being pursued by several light aircraft in a sighting which lasted 20 minutes from 11am on April 6, 1966.

The event has ever since been shrouded in mystery.

But researcher Keith Basterfield, who has spent years investigating unexplained phenomenon in Australia, said a “runaway” balloon from the HIBAL (high altitude balloon) project was the likely answer.

Each test balloon lifted a 180kg payload consisting of an air sampling and telemetry unit in a gondola and was followed by a light aircraft tasked with tracking it and triggering its 12mtr parachute via radio signal.

Immediately after the Westall “UFO” sighting, reports emerged of Government men in suits converging on the area and asking school officials and other witnesses to not talk about the event.

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A contemporary witness reported these “suits” stated what the students saw was part of a secret government exercise and that for national security reasons they were forbidden from discussing it.

“My hypothesis is that the incident involved not a UFO, but a high altitude balloon, it’s parachute and large payload,” Mr Basterfield said.

In a research paper completed this year, Mr Basterfield said a close review of all available documentation, including that searched through Freedom of Information laws, pointed to HIBAL flight number 292 as the real culprit.

“The Westall object was described as being a white/silver colour which could describe the colour of an HIBAL balloon or parachute.”

A witness account prior to the Westall sighting stated a flying object — trailing what appeared to be a long vacuum-like hose — was seen by a couple, whose surname was Frankie, near Smith’s Gully 40km north of Clayton South.

“HIBAL balloons were filled with gas through a thin tube which went to the top of the balloon and was left in place during the balloon flight,” Mr Basterfield said.

He said uncovered documents had also highlighted government concern about the potential damage if a heavy test balloon came down in a suburban area.

“The Department of External Affairs files on HIBAL reveal there was considerable discussion on possible damage to property or personnel by a HIBAL balloon,” Mr Basterfield said.

But despite government archival records showing the results of numerous HIBAL test flights, the paperwork for the launches scheduled for the day before Westall appear to have been lost or destroyed.

“What is strikingly missing is a memo reporting on the actual four launches for April 1966, one of which was scheduled for 5 April 1966, the day before Westall.

“So we have no (official) knowledge of where flight 292 went.”