A 14-year-old boy who was throwing eggs at passing vehicles has been robbed and assaulted by a motorist in Canberra's south.

Police say the boy and a friend were throwing eggs at cars on Erindale Drive in Monash last night when a white van that had been hit pulled over.

The driver of the vehicle chased the 14-year-old to Trinity Christian School where he assaulted him, demanded cash and stole his mobile phone.

ACT police are calling for witnesses.