Almost four years after he disappeared without a trace, police have begun a large-scale forensic search in the hunt for missing boy William Tyrrell.

Strike Force Rosann detectives have started a four-week search of Kendall and the surrounding bushland, where the little boy disappeared from his grandmother's front yard.

"Detectives and analysts from the State Crime Command’s homicide squad are continuing to investigate the circumstances surrounding William’s disappearance," NSW Police said.

The three-year-old was playing in the front yard at the Kendall property in September 2014, when his mother went inside for a cup of tea.