Victoria Police have reopened an investigation into a multi-million-dollar Armaguard van robbery that occurred almost 20 years ago and may have involved members of Melbourne's underworld.

The five robbers, dressed as road workers, stopped a van in Richmond in 1994 and threatened security officers before making off with $4 million.

The Purana taskforce, which investigated Victoria's gangland murders, is investigating the case, while Armaguard has offered a $100,000 reward for information that could solve the case.

Assistant Commissioner Steve Fontana says the suspects were involved in Melbourne's underworld.

"We believe that the criminals that were involved in this were very well organised and very sophisticated, and a number of the names indicated they are gangland-type individuals," he said.

Assistant Commissioner Fontana says he is confident investigators will make a breakthrough.

"When you go back and visit some of these historical cases, circumstances change for individuals, some people come forward, they talk, they might not be under the pressure they were at the time," he said.

"So we often get good results from them and we believe this is very solvable."