State Crime Commander Darren Bennett said the search, with entered the 12th day on Friday, had been difficult.
“The search continues. It is painstaking, it’s difficult,” he said.
“(There’s) no great milestone to report today except to say that the search may well be extended.
“It is obvious to all of us that we couldn’t have picked a worse time, in terms of weather. The weather has been atrocious pretty much since we started.”
Mr Bennett said he expects the search time frame, which was initially estimated at three weeks, to double to at least six weeks.
Several items found during the search have been sent for forensic testing.
While police have not specified what these are, they are believed to include pieces of clothing.
When William went missing he was wearing a Spiderman outfit.
“We just don’t know,” Mr Bennett said.
“(But) the message from the investigative team is what they’ve located so far is cause enough to keep going.”
Police have so far dug up a garden at the Kendall property, examined a concrete slab laid after William disappeared, drained a nearby creek and sifted through soil in bushland and around the home.
Specialist police divers have also searched a septic tank and water tank, and on Thursday examined a small dam on the property.
More than 15 tonnes of soil have been taken to a laboratory for analysis, but police say no DNA evidence has been found so far.