For the first time, the families of Russell Hill, 74, and Carol Clay, 73, have spoken together about the uncertainty surrounding what happened to their loved ones.

"It's hard. It's not getting any better not knowing. You can't grieve for someone if you don't know really missing or dead," Mr Hill's daughter Debbie said.

Her sister Colleen said she struggled during her daughter's birthday this year, marking the first anniversary of the last time she saw her father.

"(I was) just thinking of the things he said and what I said and the things I wish I had said," she said.

Carol's sister Jill said their disappearances didn't add up and she believed the pair were murdered.

"I immediately knew that there was something horribly wrong," she said.

"It's something that's been pre-planned, premeditated. What that raises is why would someone premeditate such a vile act on two people who were intelligent, responsible and prepared?"

It's yet to be determined what happened to the pair but investigators have long believed a third party was likely involved in their disappearance.

Missing Persons Squad detectives on Tuesday joined the families of the pair to renew an appeal for anyone with information on their whereabouts to come forward.

Mr Hill and Ms Clay were last heard from on March 20, 2020, while camping in the Wonnangatta Valley.

Mr Hill left his Drouin home on March 19 and picked up Ms Clay from her home in Pakenham in his white Toyota Landcruiser.

He was last heard from on March 20 when he made a call via HF radio saying he was at Wonnangatta Valley in the Victorian Alps.

Ms Clay told friends she was heading away and expected to return home on March 28 or 29.

Investigators have been told they were camping together at Wonnangatta River near the Wonnangatta camping ground.

Campers found Mr Hill's car with signs of minor fire damage at their campsite, which was completely destroyed by fire, near Dry River Creek Track on March 21.

All are desperate for answers and have urged anyone with information to contact Crime Stoppers.

Victoria Police want to speak to anyone who was in the Wonnangatta area about that time, including campers, 4x4 day trippers, hunters, fishermen or trail bike riders, regardless of whether they saw or heard anything.

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

Multiple searches have been conducted in and around the Wonnangatta Valley during the past 19 months, including with cadaver dogs.

Australian Associated Press