Mr Morrison said police would "never stop" looking into the case after the three-year-old vanished seven years ago, and that it was a great statement about the dedication of police.

"My late father was a policeman so I have a bit of an inkling about the level of patience and determination required from law enforcement to never give up," Mr Morrison told reporters on Thursday.

He thanked police for their "extraordinary work", which he said had led to "huge inroads" which he hoped would give William's family some comfort.

Police are digging in the NSW mid-north coast town where William was last seen in 2014, with a particular focus in the garden of a Kendall home that belonged to William's foster grandmother.

NSW Police revealed on Wednesday they'd seized a car that belonged to the foster grandmother, who has since died.

The grey Mazda was taken from a home in Gymea in Sydney's south under a coronial order last week and is undergoing extensive forensic examination.

Also on Wednesday, William's foster parents were charged with the unrelated assault of a different child and will face Hornsby Local Court on Tuesday.

Meanwhile, the former homicide detective who was initially in charge of the case on Thursday defended his handling of the investigation.

Retired homicide detective Gary Jubelin was removed from the case after four years.

Police Commissioner Mick Fuller this week said the new investigation team had "inherited what was a bit of a mess".

Mr Jubelin took issue with this, saying he provided monthly progress reports to his superior officers detailing everything - "what suspects I was targeting, what the future directions were".

Mr Jubelin admitted he had formed a friendship with William's foster parents and believed the foster mother to be "a very decent human being".

However, he said he "went hard" when investigating the couple.

"I basically ambushed the (foster) parents and then I interrogated the (foster) parents," he told Sydney radio 2GB on Thursday.

Mr Jubelin eliminated them as suspects after a covert operation that included placing a listening device in their car.

"At the time I was taken off the investigation ... I was certainly of the belief that they were not involved," he said.

He investigated all theories, including that William had died in an accident, but he said any theory had to be backed up with facts.

The investigation is now considering whether William might have died after falling from the balcony of the foster grandmother's house.

Mr Jubelin said the timing of the release of the information about the foster parents during a high-profile search of the Kendall home was "a big coincidence".

"We've all got to be a little bit careful," he said.

Some 30 to 40 people are helping with the search, including officers from NSW Police and the Australian Federal Police, as well as Rural Fire Service volunteers.

Police have turned their attention to an area of bushland about a kilometre from the house where William was last seen.

A mechanical digger has been used to remove the topsoil, and officers are using ground-penetrating radar and 3D cameras to analyse the ground.

The findings of a coronial inquest into William's disappearance, which concluded last year, are yet to be handed down.

A $1 million reward for information on the case still stands.

Australian Associated Press