Tyrrell disappeared in 2014 at age 3, and was last seen at a property owned by his former foster grandmother.
It emerged last year that William's foster mother and now-deceased foster grandmother were persons of interest in his disappearance.
They have both denied any involvement and no charges were laid.
Due to an extensive, but ultimately unsuccessful initial search for the boy, police began operating on the belief he was kidnapped.
The case gained national attention and sparked a tireless years-long search to find him.
Last year police launched a renewed effort, scouring bushland around the property where Tyrrell was last seen.
Around 30 police officers spent more than a month searching scrub and waterways near Kendall, south of Port Macquarie, on the NSW mid north coast.
AFP officers also used ground-penetrating radar to scan the garage of the property and in nearby bushland to detect anything out of the ordinary.
Teams searched rainwater and septic tanks at the property and police divers entered a nearby creek bed.
During the search, officers found pieces of cloth seeming to match the spiderman outfit Tyrrell was wearing when he disappeared, and even bone fragments. However, nothing was deemed to be connected to the case.
A $1 million reward remains in place for information that leads to the recovery of William Tyrrell.
