Northern Territory police have arrested a 40-year-old man for smashing a watermelon over a bus driver's head in Darwin's northern suburbs.

Police say the attack began when the driver asked the man to pay his bus fare.

Watch Commander Debbie Gabolinscy says the man got off the bus after the watermelon attack but then got back on and continued the assault.

"Fortunately a couple of passengers interjected and it appears the driver has only suffered some soreness and pain," she said.

The man has been arrested and police expect to lay charges later today.