Carol Clay, Russell Hill theories emerge a year after couple went missing in Wonnangatta Valley

Carol Clay, 73, and Russell Hill, 74, had been in a relationship for years when they set out on their camping trip to the picturesque valley. Photo / Supplied

It's been 12 months since Russell Hill and Carol Clay left their homes in Victoria for a camping trip in the tranquil but remote Wonnangatta Valley.

The couple, who had been having an affair for decades, were last seen on March 20 last year.

After a year of investigations, with police leaving no stone unturned, detectives believe they could be close to a potential breakthrough in the mysterious case.

Speaking to A Current Affair, Detective Inspector Andrew Stamper said police had a number of people in their crosshairs, including the driver of a white ute.

"It might seem like only a very small possibility that those in the white ute will have information about Russell and Carol's disappearance but we can't afford to leave any stone unturned," he told the programme.

The vehicle was seen at a public toilet in the valley the day before the pair last made contact with friends.

Police are also investigating if the couple were ambushed early considering the toilet at their campsite had been unused, suggesting the pair had only spent a short amount of time in the area before their deaths.

Hill called his friends using his high frequency radio on March 20, with police believing the couple might've been killed shortly after.

In what's become one of the most baffling missing persons case in Australian history, Stamper said he had doubled his investigation time.

The latest theory police are following is if the couple were shot by deer hunters.

Police have speculated if the couple witnessed something they weren't supposed to, perhaps illegal hunting, and tragically paid for it with their lives.

Carol Clay told friends she was going camping and would be back on March 29. Photo / Supplied

Hill's $2000 drone, which he regularly used while out camping, is still missing, with police questioning if the hi-tech gadget inadvertently filmed something illegal.

"There's a number of scenarios we are looking at," he said, referring to the couple having a run-in with hunters.

"Certainly that's one we can't eliminate. I'm not wedded to any specific scenario."

Police hoped they might've stumbled on a breakthrough in the case last week when a drone similar to Hill's was found in the valley and handed to officers in East Gippsland, but the drone was not a match.

Hill, 74, and Clay, 73, had been in a relationship for years when they set out on their camping trip to the picturesque valley.

But in a horrifying twist, the couple's campsite was found burnt to the ground with their tent destroyed and their sleeping bags missing.

One of Hill's close friends, who went camping with the 74-year-old over the years, offered a sinister theory as to what might've happened to the pair.

Speaking on Nine's Under Investigation, friend Rob Ashlin said he didn't have much hope the couple would be found.

"The fact that it's been reported that those sleeping bags were missing, makes me feel that those sleeping bags were carted out of there and ... used as body bags," Ashlin said.

"There are a lot of places – the country is very rugged – where they can be got rid of, never to be found again."

Ashlin also knew his friend wouldn't set up the campsite in the way it was found, with items scattered around everywhere.

"I knew straight away in my own mind that something really unforgiving had happened," he said, after he had seen photos of the destroyed campsite.

Ashlin's theory was supported by Lachlan Culican, who assisted police during their difficult search of the terrain.

Culican works as a high country musterer and is an expert in navigating that part of Australia. He also believes the couple might've stumbled upon something illegal or had a run-in with a deer hunter.

During his search with police, Culican was stumbling upon a dead deer every 200 or 300 metres.

Hill left his Drouin home in Victoria's Gippsland region on March 19 last year.

He travelled to pick up Clay from her Pakenham home, in the southeast of Melbourne, before the pair travelled in Hill's Toyota Land Cruiser towards Wonnangatta Valley.

The pair spent a night in Howitt High Plains before driving into the valley's rugged terrain.

Hill called friends on March 20 at 6.30pm over his radio and told them he was camping in the valley. Clay told friends she was going camping and would be back on March 29.

The next day, at 2pm, other campers found the couple's campsite burnt down, with Hill's car sitting nearby.

Detectives have long been confident the couple were not involved in a murder suicide and have found no evidence the couple attempted to fake their own deaths or even leave the campsite.

Police believe the campsite was burnt down in an attempt to destroy any potential forensic evidence.