Police have narrowed their search for missing boy William Tyrrell, who disappeared on the New South Wales Mid North Coast close to four years ago.

The little boy was last seen playing in his grandmother’s yard at Kendall in September 2014, becoming one of the country’s biggest missing persons case.

The then three-year-old disappeared in a matter of moments, sparking a 10-day search of the home and nearby bushland.

18 square kilometres of land was searched after he vanished in Kendall in 2014.

50 officers are now focusing on three square kilometres, with much more detail, to ensure nothing was missed.

Detective Chief Inspector Gary Jubelin tells Macquarie Radio’s Chris Smith the decision for a new search comes from “an investigative point of view”.

He says the initial focus of the search, while extensive, “quite rightly so, was on finding a lost boy”.

This second search is being considered a “forensic search” with a view of deliberate human intervention.

“I need to be able to show definitively that William’s disappearance was not the result of misadventure but as a result of human intervention.”

Detective Chief Inspector Jubelin, who has been on the case since the beginning, tells Chris “letting go” is “just not an option”.

“I believe someone out there has information and if they’re sitting on that information they’re taking a real risk that they’re committing a criminal offence by concealing the offence.”

Listen to the full update below

A large-scale forensic search is underway on the mid north coast, where little boy William Tyrrell vanished from his grandmother’s yard at Kendall in 2014. New search could last four weeks @2GB873 pic.twitter.com/9YdbqDCDay

— Alice Hogg (@alicemhogg) June 12, 2018