Police have opened a fresh search for missing boy William Tyrrell in bushland on the mid-north coast of NSW, almost four years after he disappeared from his grandmother’s yard.

Detectives on Wednesday will begin a four-week forensic sweep around the property where he was last seen in Kendall, near Port Macquarie on September 12, 2014.

The initial search focused only on finding William and was not carried out with a view of deliberate human intervention, NSW Police said in a statement.

William was three years old when he vanished while playing in the yard at his grandmother’s home.

Hundreds of locals and emergency services workers spent 10 days looking for him in the rural township, forests, creeks and paddocks, but no trace of the boy was found.

Detectives have been investigating the circumstances of his disappearance for the past four years, launching a special strike force led by the homicide squad.

The new large-scale search will be co-ordinated by search experts from NSW Police’s public order and riot squad.

In 2016, the NSW government announced an unprecedented $1 million for information leading to William’s recovery.

On the third anniversary of William’s disappearance, NSW detectives investigating the case said they were determined to find him.

Detective Chief Inspector Gary Jubelin, who’s leading the investigation, said “hundreds” of persons of interest had been eliminated in the past year.

“I can assure you that we haven’t given up on the investigation, I can assure you we will not give up on the investigation,” he told reporters in September last year.

“The investigation is currently very active.”

William’s foster family at the time spoke of their “unspeakable heartache” and vowed to never give up searching.

“We will never stop looking for you, and until the tomorrow we yearn for comes, we will never give up hope that you will be found and returned home to the arms of your loved ones where you belong,” they wrote in a Facebook post.

-with AAP