A young woman has been punched in the mouth during a violent carjacking attempt in Melbourne's north-west moments after the assailant robbed a milk bar armed with a screwdriver.

Police are appealing for the public's help to identify the wanted man, described as being of African appearance, after his violent rampage in Brunswick West about 7.55pm on Tuesday.

Fawkner Embona Taskforce investigators said the man was armed with a screwdriver when he entered a milk bar on Moreland Road and walked behind the counter.

Once inside, they said he armed himself with a bat and demanded cash from the male attendant, who ran from the store.