His disappearance - and suspected murder - has been the focus of an ongoing Victoria Police missing persons investigation but without success.

Now a $1 million reward is being offered for information to help solve the cold case.

Detective Acting Inspector Tony Combridge of the Missing Persons Squad said police believe Mr Falos was murdered.

"It may be 18 years since Nick disappeared, however, I would like to strongly reinforce to the community that this is very much an active investigation," he said in a statement on Wednesday.

"While Nick is no longer believed to be alive, we are still hopeful of finding out what happened to him.

"It is also our belief that a number of people were potentially involved in Nick's murder."

Mr Falos, 58, of Thornbury, was last seen on Friday, March 14, 2003, around 8.30am as he drove his Porsche along Albion Street in Brunswick.

The car was found burned out in the Yarra State Forest, near Gladysdale, three days later on March 17.

He was reported missing on April 1 by a neighbour who became concerned because they hadn't seen him for several weeks.

Police searched bushland near Gladysdale in May that year but found no trace of Mr Falos.

Police are investigating a number of lines of inquiry including a phone call made from a payphone to Mr Falos on the morning of Saturday, March 15, 2003.

They have also looked at possible links between his disappearance and reports of cannabis cultivation in the Yarra State Park and Yarra Junction areas.

Mr Falso, who was of Greek heritage, also went by the name Nikolaos Falieros.

In 2018, a 77-year-old Endeavour Hills man and an 84-year-old Devon Meadows man were arrested in relation to his disappearance.

Both were later released without charge.

Police are keen to hear from anyone who recalls seeing Mr Falos or his red Porsche in the Yarra Junction area on March 15 or 16, 2003.

Australian Associated Press