William Tyrrell search: Foster parents leave Sydney for Mid-North Coast

William Tyrrell's foster mother has been identified as the sole person of interest in the toddler's 2014 disappearance. Photo / Supplied

William Tyrrell's foster parents have left Sydney as police continue to search for the missing boy's remains.

The couple, who cannot be named for legal reasons, have spent the week on the Mid-North Coast.

They were seen walking along the coastline as police spent a 19th straight day trying to find any signs of William.

His foster mother has been identified as the sole person of interest in the toddler's 2014 disappearance.

William Tyrrell's foster mother has been identified as the sole person of interest in the toddler's 2014 disappearance. Photo / Supplied

Police Commissioner Mick Fuller praised the work of investigators working through constant rain.

"They've found material and other exhibits of interest but obviously they have to be analysed and DNA-tested. Because of some of the quality or state of these exhibits, it just does take time," he said.

Fuller said he hopes the case could be solved before he retires at the end of January.

Strike Force Rosann was established to find the boy, who disappeared more than seven years ago from his foster grandmother's home at Kendall.

William was playing with his 5-year-old sister in the yard at his foster grandmother's house on Benaroon Dr, Kendall, on September 12, 2014, when she suddenly couldn't find him.

Police look at a water tank at the house William Tyrrell went missing from near Kendall. Photo / Supplied

He was 3 years old at the time and wearing a Spider-Man costume.

NSW Police returned to the Mid-North Coast property to launch a fresh search based on what they have said is new information in the case.

Officers have established a search area about a kilometre from the house, where they have been scouring the creek bed and surrounding bushland for evidence.

A replica pistol was found in bushland on the NSW Mid-North Coast on Tuesday.

Missing child William Tyrrell. Photo / Supplied

Police discovered the gun, which is not thought to be linked to the missing child, in cleared bushland beside Batar Creek Rd at Kendall, less than a kilometre from the former home of William's late foster grandmother.

Police had been probing whether the 3-year-old fatally fell from the balcony of his foster grandmother's home.

A cadaver dog was used to comb through a garden bed.