A MAJOR investigation into one of the most depraved sexual assault cases on the NSW mid north coast has led to the arrest of a man who allegedly had sex with cows and encouraged a young boy to do the same.

A 53-year-old Toormina man was arrested outside a business near Coffs Harbour this morning and charged with 21 offences including bestiality and aggravated sexual assault.

Police will allege the man befriended the boy, then aged 13, at a local business in 2003.

Over the next two years the man allegedly committed four acts of bestiality involving cows and encouraged the teen to participate.

Following the mans arrest, a search was carried out at a home at Toormina.

Police seized a computer and other electronic items for examination.

The man has been granted conditional bail and is expected to appear in the Coffs Harbour court on Monday, February 10.