Police now believe it unlikely Russell Hill and Carol Clay are still alive.

But the number of investigators on the case has been increased and optimism remains Mr Hill and Ms Clay's families will be given the answers they desperately need, Detective Inspector Andrew Stamper said on Friday.

Relevant people detectives want to contact them include campers, 4x4 day trippers, hunters, fishermen or trail bike riders, regardless of whether they saw or heard anything.

On Saturday it will be one year since the campers, who are in their 70s, vanished from Wonnagatta in Victoria's high country.

Mr Hill, 74, left his Drouin home and picked up Ms Clay, 73, from her home in Pakenham in his white Toyota Landcruiser.

He was last heard from the next day via HF radio, stating he was at Wonnangatta Valley in the Victorian Alps.

Detectives want to speak to anyone who was in the area of Howitt Plains and Zeka Spur Track on March 19 or 20, and the Wonnangatta Valley and Wonnangatta Station between March 20-24.

Campers found Russell's vehicle with signs of minor fire damage at their campsite, which was completely destroyed by fire, near Dry River Creek Track in the Wonnangatta Valley on March 21.

Police would like to speak to anyone who saw this fire or the smoke from it.

Investigators have been told Mr Hill took his DJI Mavic drone with him on the trip and the drone is still yet to be recovered.

A drone was handed into police earlier this month but has been confirmed not to have belonged to Mr Hill.

Police also have appealed for information about a white dual cab ute seen near the pair's campsite on March 19 last year.

Investigators have not been able to account for the vehicle but do not believe it is connected to the pair's disappearance.

Detectives have established Mr Hill was camping alone with his Landcruiser in the area of the King Billy and Bluff Track between March 11 and 13.

An older person or pair were later seen in the Black Snake Creek, Eaglevale River crossing and the Ollies Jump area on March 22 and 23 but investigators have never been able to establish if this was Mr Hill and Ms Clay.

Detectives are calling for anyone in this area, including the pair themselves, to come forward.

It is still to be determined whether the pair's disappearance is suspicious.

Ms Clay had told friends she was expecting to return by March 29.

"The grief that these families have had to deal with over the past year, coupled with immense public speculation, is immensely difficult to comprehend and our thoughts are with them on this anniversary," Det Insp Stamper said.

"I know they will be hanging on every phone call and every knock on the door, hoping they will finally get some answers.

"Sadly, at this stage we believe it is highly unlikely that either Russell or Carol are still alive."

Australian Associated Press