A good Samaritan who tried to help a woman who looked unconscious in East Melbourne has been attacked and robbed by the person she was trying to help.

The 23-year-old woman was walking down Powlett Street on Sunday morning when she noticed a female lying face down in a driveway.

When she stopped to help, the woman stood up with a blunt weapon and demanded her wallet.

The unknown woman then chased the victim, who flagged down passing cars.

Detectives are also investigating another armed robbery committed moments earlier in nearby Richmond.

Two women in their 20s were threatened by an an unknown man and woman with weapons, believed to be knives.

One of the women surrendered her handbag.

Detectives believe the two incidents may be related, and are urging witnesses to come forward.

AAP