Fresh leads in search for missing married man and his secret lover as police call in expert help from overseas - and say they're closing in on solving the mystery

The search for two secret lovers who vanished without a trace on a remote camping trip six months ago has gone international, as detectives turn to overseas and interstate experts for assistance.

Russell Hill, 74, and Carol Clay, 73, went missing on March 19 in the Wonnangatta Valley in Victoria's East Gippsland region. 

Their disappearance has baffled authorities, who found their campsite burned to the ground and Mr Hill's car slightly damaged by fire two days later. 

Detective Inspector Andrew Stamper from Victora Police's Missing Persons Squad told the Herald Sun he couldn't divulge any details about the nature of the global assistance - but said cops were getting closer to solving the mystery. 

'We've learnt a lot along the way. We know a lot more than we did six months ago,' Insp Stamper said.

Russell Hill, 74, and his friend Carol Clay, 73, went missing during a camping trip on March 19 in the Wonnangatta Valley in Victoria's East Gippsland region 

Mr Hill's white Toyota Landcruiser was found with minor fire damage at their burnt campsite near Dry River Creek Track in the valley on March 21

Last month police revealed they were exploring a theory that the couple were attacked in their sleep.

Detectives believe the potential attackers may have torched their campsite to destroy forensic evidence of a crime. 

The pair were in a secret relationship and didn't tell anyone where they were going. Ms Clay just told friends she would be gone for a few days. 

In June, investigators said three theories about Mr Hill and Ms Clay's disappearance were being probed. 

Foul play is strongly suspected on the balance of probabilities, but there is no evidence and no suspects.

Another possibility is that the couple were flying Mr Hill's $2,000 drone and died of exposure after getting lost off the path looking for it.

The third is that they died lost in the bush for any reason, but have not been found because wild dogs ate their remains.

Police are confident the pair did not fake their own deaths and do not believe it's case of murder-suicide.

The search for Mr Hill and Ms Clay has gone international, as detectives turn to overseas and interstate experts for assistance

The pair went missing in the Wonnangatta Valley, more than 200km north east of Melbourne

The key to unlocking the case may have gone up in flames in a fire that destroyed their campsite. 

Mr Hill's Toyota four-wheel-drive had been found by local police next to his burnt-out tent, but the pair were gone without a trace.

The vehicle itself had also sustained damage in the fire, but was still able to be driven away from the scene. 

At the time, police had believed the fire was probably started by a dodgy phone charger or campfire spark and the couple had simply been lost in the forest. 

But the fire could also have been lit by a thief rummaging around in the tent  - or by a murderer trying to cover their tracks. 

A spare set of keys was found hidden on the car, but it is believed Mr Hill took the main set with him.

That the car was locked likely indicated the pair left the campsite voluntarily, probably to go for a walk or fly the drone.

The drone remains missing despite an extensive search that ended on April 6 without anyone finding a trace of them. 

Russell Hill hangs onto a drone that he had with him when he went missing. The drone has not been found despite an extensive search of the area 

A drone similar to the one that remains missing

If wild dogs had found the pair before police, their remains could have been eaten and scattered to the point they would be missed by searchers.

Dogs would have had plenty time to scavenge on the bodies because the pair were not reported missing for some days after their likely deaths. 

Mr Hill had left his Drouin home on March 19 for a camping trip along the Dargo River in Victoria's northeast and planned to leave the region on March 26. 

Ms Clay, meanwhile, had told friends she was going away for a few days and expected to be home by March 28 or 29.

Mr Hill had been a keen amateur radio enthusiast and made his last broadcast from the bush on March 20.

'This place is in the middle of nowhere,' the source said. 'It's just problematic in the extreme.' 

Police are now certain the couple died within 18 hours after Mr Hill's last message, as a passing hiker saw the burned-out campsite about 2pm the next day.

Because the pair weren't due back until up to a week later, the alarm wasn't raised for days and precious time was lost.

The police source said despite the delay in the missing person squad being brought into investigate the case, the fire ravaged camp site was thoroughly investigated at the time. 

Personal belongings were left in Russell Hill's vehicle, which sustained fire damage when his tent went up in flames

Carol Clay, 73, who was once the President of the Country Women's Association of Victoria, had been in a relationship with Russell Hill for many years in the lead up to their disappearance

In May, bizarre details emerged of an 'oddball loner' living in the Victorian Alps, who has been questioned over a number of mysterious disappearances.

Known as 'Buttons' or 'the Button Man', the expert bushman became a person of interest after concerns about his odd behaviour was raised by locals.

He is understood to have earned his nickname due to his hobby of using deer antlers to make buttons - which he then uses as large ear piercings. 

'Being creepy is not illegal,' a police source told Daily Mail Australia at the time.  

The source confirmed there was nothing to indicate the mysterious loner had killed the couple.

'We don't think they're in Queensland living (a secret) life. They're too old really to drop off the grid and why would they?' the source said. 

Police believe it is more likely than not that the pair met with foul play while out in the remote wilderness. 

The area itself is considered an ideal spot to commit murder. 

A new image of Mr Hill's white Toyota Landcruiser. It was found with minor fire damage at their burnt campsite near Dry River Creek Track in the valley on March 21

Victoria Police Missing Persons Squad Detective Inspector Andrew Stamper is leading the investigation into the mystery 

Phone reception is practically nil and unlike some other areas popular with hikers, the remote ­Wonnangatta Valley does not see a lot of foot or road traffic.  

While cold-blooded murder was an avenue of investigation, detectives remain open to all kinds of scenarios.

'There are a lot of people who go up there and do illegal stuff. Like people who go hunting in the national park and ride motorcycles. Has he had a dispute with one of them that turned bad?' the source said.  

The pair had been carrying out a secret affair for years behind the back of Mr Hill's wife Robyn, who has described Ms Clay as a 'long time family friend'.  

While the relationship was largely hidden from Mr Hill's devastated wife, it has been well known to police since the pair were reported missing by Mrs Hill days after her husband last made contact via radio. 

When questioned about the couple's relationship in April, Victoria Police Missing Persons Squad Detective Inspector Andrew Stamper skirted around the question.  

'It is delicate and we are being very respectful of both families wishes and concerns here,' he said at the time.

Russell Hill and Carol Clay were last heard from on March 20, with Mr Hill's wife unaware he was travelling with another woman

'Both of these people have large and loving families who are grieving at the moment, and they want answers about where their loved ones are.'

While detectives probed the couple's relationship, it was quickly ruled out as a likely reason for their mysterious disappearance.  

Police are desperate for people who may have been in the area at the time to let them know so that they can be ruled out of the investigation. 

Detectives are also keen to learn if anyone may be hanging onto dash cam footage that could help identify other leads. 

It is understood the case remained 'very wide open'. 

'It's a genuine mystery,' the source said. 

Anyone with information is urged to contact CrimeStoppers on 1800 333 00