A 55-year-old Caroline Springs man is being questioned following a dramatic arrest by specialist police at a remote campsite in Arbuckle Junction, some 280km northeast of Melbourne, on Monday night.
The man, a check captain with budget airline Jetstar, remains in custody, while his Nissan Patrol four-wheel-drive has been seized.
He has been stood down by the airline following his arrest.
"Jetstar has been advised by Victoria Police that one of its employees is under investigation for a serious crime and will work to assist in this due process any way we can," a spokeswoman confirmed.
"As a matter of course, the employee has been removed from duty as a result of their arrest."
Mr Hill and Ms Clay were last heard from on March 20, 2020, while camping in the Victorian Alps.
Mr Hill left his Drouin home on March 19 and picked up Ms Clay from her home in Pakenham in his white Toyota LandCruiser.
On March 20, Mr Hill made a call via high-frequency radio saying he was at Wonnangatta Valley.
During the call, the experienced outdoorsman said he was having radio transmission issues.
Ms Clay, a former Victorian Country Women's Association president, told friends she was heading away and expected to return home on March 28 or 29.
Investigators were told the couple were camping together at Wonnangatta River near the Wonnangatta camping ground.
Campers found Mr Hill's fire-damaged car at their campsite, which was destroyed by fire, near Dry River Creek Track on March 21.
The pair have not been seen or heard from since.
Multiple searches have been conducted in and around the Wonnangatta Valley, including with cadaver dogs.
Earlier this month, police released images of a blue four-wheel-drive, believed to be a mid-to-late-1990s model Nissan Patrol, taken by a safety camera in the area at the time Mr Hill and Ms Clay went missing.
Witnesses reported a number of sightings of the vehicle, including near the campsite, with police yet to track down the driver or exclude them as a person of interest.
Police believe the pair may have had an altercation with another camper, possibly the driver of the blue 4WD.
It was towing a trailer and was heard performing a 20 to 30-point turn on a narrow track the night the pair vanished.
Mr Hill's sister Jill knew the pair were in a secret relationship despite Mr Hill being married.
She believes that was part of the reason they travelled remotely together for their trips.
The families have been desperate for answers and recently gave television interviews on the uncertainty surrounding what happened to their loved ones.
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