Police share details about the fresh search for missing Sydney boy William Tyrrell.

The fresh search for William Tyrrell in bushland on the NSW mid-north coast will focus on finding evidence to show his disappearance was the result of human intervention and not misadventure, police say.

For the next four weeks, teams of experts and hundreds of emergency services workers will conduct a sweep around the Kendall property near Port Macquarie, where the three-year-old was last seen on September 12, 2014.

Lead investigator Detective Chief Inspector Gary Jubelin told media at Kendall on Wednesday authorities hold “grave fears” over finding William alive nearly four years after he vanished.

“It has been a very long time,” he said.

The initial search focused only on finding “a little boy lost”, Det Insp Jubelin said.

“The purpose of this is that so if we present evidence to a court, we can show beyond reasonable doubt that Williams disappearance was a result of human intervention and not through misadventure.”

William was playing in the yard of his foster grandmother’s home when he vanished.

The new search will coincide with William’s seventh birthday in two weeks’ time.

Det Insp Jubelin believes people do know what happened to the boy and reiterated the $1 million reward for information leading to William’s recovery.

“I strongly believe that there are people out there who have information on this and I want to make a point to those people that if you do have information concerning what happened to William, you are committing an offence if you do not come forward.”

He confirmed there were still persons of interest, but would not detail how many or who they were.

William’s foster family have never given up hope of finding the boy, and on Wednesday posted a message to social media.

“Today marks 1370 heartbreaking days since William disappeared,” it read.

“William’s loved ones are, and will forever be indebted to the tremendous ongoing commitment of NSW Police; in particular, Strike Force Rosann who have been absolutely relentless in their ongoing investigation in the search for William.

“Since that tragic day on Friday 12 September 2014, hearts across Australia have been broken, but we have and will continue to maintain unwavering faith in NSW Police and carry hope in our hearts that William will be found. He is in our hearts always … Always”

The new large-scale search is being co-ordinated by search experts from the NSW Police public order and riot squad.

Listen to The Quicky debrief on the truth about William Tyrrell’s parents, and what happened after the three-year-old’s disappearance. Post continues below.

Timeline of the disappearance of William Tyrrell

2014

SEPTEMBER 12 – Dressed in a Spiderman outfit, three-year-old William Tyrrell goes missing from the front yard of his foster grandmother’s home in Kendall, 40km south of Port Macquarie.

SEPTEMBER 21 – Police stop searching for the missing boy after scouring surrounding bushland and neighbouring houses.

2015

JANUARY 20 – Police search the home and business of washing machine repairman William Spedding, who had been due to carry out repairs at the Kendall house at the time the boy disappeared.

JANUARY 23 – Spedding publicly denies any involvement in William’s disappearance and no charges have been laid against him.

FEBRUARY 19 – Homicide detectives take over the case and say it’s likely William was abducted.

MARCH 2 – Police search bushland near Bonny Hills for three days after a tip-off.

APRIL 17 – William’s foster parents speak publicly for the first time in an emotional video released through police which does not identify them.

APRIL17 – Police say the boy may have been a victim of a pedophile ring.

SEPTEMBER 12 – “Where’s William” week is launched one year after he disappeared.

2016

SEPTEMBER 12 – A $1 million reward is offered for information leading to William’s return.

2017

AUGUST 24 – William’s foster child status is revealed after a court ruling.

2018

JUNE 12 – NSW Police announce the start of a four-week forensic search of bushland in Kendall.