A woman was carjacked as she waited outside a laundromat in Melbourne's south-east this morning, with a Good Samaritan who tried to help also being threatened.

The 44-year-old woman from Parkdale was waiting inside her car on Chandler Road in Keysborough at around 6am, when she was ambushed from behind by a thug with a knife.

She was pulled out of the driver's seat, and she was dragged from the car as she attempted to fight back.

BUDGET: Could the government's tax reform fail before it even begins?

A man ran to try to intervene, but a second person with a knife emerged and threatened him.

The pair then took off in the woman's car.

Witnesses said they were shocked by the speed and intensity of the incident.

"That poor girl was absolutely traumatised," Linda Widdup said.

HANTAVIRUS: Another passenger positive as hantavirus cruise ship heads to Netherlands

"Luckily, there was a young fella up the road, only a little but further up that heard her screaming, and he came running down."

The ordeal lasted less than a minute, but detectives are now trying to piece together exactly what happened, and where the thieves might have gone.

Neither the woman nor the Good Samaritan received any physical injuries, but the victim was shaken up by the experience.

"This poor girl's just gone to do some laundry, and look what's happened," Widdup said.

NEVER MISS A STORY: Get your breaking news and exclusive stories first by following us across all platforms.

• Download the 9NEWS App here via Apple and Google Play

• Make 9News your preferred source on Google by ticking this box here

• Sign up to our breaking newsletter here