The renewed search for the remains of missing three-year-old William Tyrrell could take months, with investigators on day six having scoured only a fraction of the scrub near where the pre-schooler disappeared.

As the end of the first week of the “high intensity” search draws near, police have searched an estimated 10 to 20 per cent of the areas of interest – or about one kilometre squared.

At that rate, the painstaking effort could take up to nine weeks to complete.

Police have so far combed bushland and drained a creek in an area a kilometre from the Kendall property where the boy’s foster grandmother lived and where he was last seen seven years ago.

A hessian bag was found and collected for forensic testing on Saturday, with at least two other pieces of fabric placed in evidence bags and sent off earlier in the week.

Investigators have also scanned a concrete slab at the property, laid after the boy disappeared, but have since confirmed nothing of note was found.

Strike Force Rosann officers are testing theories William may have fallen from a balcony at the property, sifting through a garden below. Earlier this week, police seized a Mazda that previously belonged to the foster grandmother, who has since died.

It comes after police this week also charged the boy’s former foster parents over an unrelated alleged assault on a different child.

The pair is due to face court at Hornsby on Tuesday.

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.

-AAP