A MAN has threatened to slash a baby's throat during an attempted highway robbery.

After flagging down a car on the Newell Highway, in the state's central west, the man walked up to the car while holding a knife to the neck of a baby he was carrying.

Brazen ... a computer-generated likeness of the man.

The 36-year-old woman driving the car, about five kilometres south of Parkes on Monday night, told police that the man had stood at the driver's side window and demanded she hand over money.

The woman drove off and went to Parkes police station to report the encounter.