A man and a woman have been arrested and charged after they allegedly wielded a chainsaw in northern NSW and threatened a 17-year-old boy as he hid inside his house.

The incident, which happened in broad daylight at Grafton, unfolded just after noon on Friday.

Police said the teenager had just returned to his home on Bent Street when he found two girls, 16 and 17, waiting for him.

He allegedly grabbed a knife and threatened the younger girl, causing her to run out of the house and alert a woman, 44, who was waiting on the street in her car.