Russell Hill and Carol Clay have been missing for 19 months, with police conceding its unlikely the pair are still alive.

On Monday, family spoke publicly for the first time.

Ms Clay's sister Jill called on their killers to come forward.

"Every day it's like wearing a big heavy overcoat, it weighs you down, makes you feel heavy and sad," she told Seven News.

"You've undertaken a heinous act... it's time to turn yourself in."

Mr Hill's daughter Debbie told Seven News "it's really hard not knowing" what happened to her dad.

Inspector Andrew Stamper said the killer should feel "very uncomfortable" as police close in.

Police believe Mr Hill, 74, left his Drouin home on March 19, 2020 and collected his friend Ms Clay, 73, from her home in Pakenham in his white Toyota LandCruiser.

They travelled through Licola, spending one night at Howitt High Plains before heading into Wonnangatta Valley.

Mr Hill was last heard from on March 20 via HF radio, stating he was at Wonnangatta Valley.

Campers found Mr Hill's car with signs of minor fire damage at the duo's campsite near Dry River Creek Track in the Wonnangatta Valley the next day.

Their campsite had been destroyed by fire.

A number of searches have been conducted in and around the Wonnangatta Valley during the past 12 months, including on April 4 this year with cadaver dogs.

Australian Associated Press