A woman who tried to help a dying motorcyclist has herself been killed by a passing vehicle, in a tragic double fatality in Melbourne's east.

Key points: A 36-year-old man riding a motorbike collided with a car just after 10:00pm

A 36-year-old man riding a motorbike collided with a car just after 10:00pm A 27-year-old woman who went to help him was hit by another vehicle

A 27-year-old woman who went to help him was hit by another vehicle Both died at the scene

The 36-year-old man was thrown from bike and died after a crash with a car at an intersection in Wantirna, just after 10:00pm last night.

A 27-year-old woman, who witnessed the accident and went to help the rider, was also killed when she was hit by another car.

Major Collision Investigation Unit Acting Detective Inspector Brad McArthur said the first car had been turning at the intersection when a distant motorcycle had "come up on him so fast, and the next thing he knew was that the motorcycle struck his vehicle".

"And it's struck with such force that it's spun his car around 180 degrees."

He said the motorcyclist was travelling at an "absolutely irresponsible and reckless" speed that was well above the 80-kilometre-per-hour limit.

The woman who was killed had two children under 10, as well as a partner, he said.

"The effects of this crash will affect a huge number of people for the rest of their lives."

He praised her efforts as "heroic".

"She was trying to do the right thing," he said.

"She saw somebody that was in need of help, and she's gone out of her way to help, but unfortunately it's led to her death.

"She's a hero as far as I'm concerned.

"We overuse words like tragedy … but it really is. In this case, these words ring true. It is an absolute tragedy."

The 36-year-old motorcyclist died at the scene. ( ABC News )

Acting Detective Inspector McArthur said that the motorcyclist was known to police and "evidence of drugs" in the form of "white powder", plus "an amount of cash", was found on his body.

Both victims were Wantirna residents.

Acting Sergeant Alistair Parsons described it as "a very, very nasty crash", and said the Major Collision Investigation Unit had been at the scene all night.

"They've been going over the scene to try and get to the bottom of exactly what's happened," he told ABC Radio Melbourne.

"What they do need is witnesses."

Acting Sergeant Parsons said police were keen to talk to anyone who had dashcam footage of the vehicles before the accident, at the intersection of Boronia Rd and Amesbury Ave.

He said police were also investigating the state of street lighting at the scene, but added that the position of the smashed vehicle and the colour of the woman's clothing may have also caused visibility issues.

Neither driver was injured, and police said both men stopped to assist.