Officers were called to the on-ramp of the Monash Freeway at the Eastlink interchange in Dandenong North about 9.30am on Thursday, following reports of an armed man in the emergency lane.

Upon arrival, they tried to negotiate with the 53-year-old and calm him down before he produce a knife and advanced on them.

The officers fired a non-lethal beanbag round to try to stop the man before shooting him twice in the chest with a semiautomatic firearm.

The Narre Warren man died at the scene.

Victoria Police Assistant Commissioner Bob Hill said the man had been suffering a "mental health episode" but the full circumstances were unclear.

"Whether it was a matter of self-harm or harming others ... the male involved was quite distressed," he told reporters on Thursday.

"We are still trying to understand why, what's motivated, what's triggered this situation."

The assistant commissioner stressed it was a sad occasion for the man's loved ones and the community at large.

A police officer suffered minor injuries.

Homicide detectives will investigate the incident with oversight from the Professional Standards Command, as is standard procedure following a police shooting.

Footage from body-worn cameras activated by officers, as well as freeway cameras, will be used in the investigation while police have also appealed for phone or dashcam footage from motorists.

Police Association secretary Wayne Gatt said police involved in the incident had been receiving support.

Australian Associated Press