A man questioned over the disappearance of toddler William Tyrrell was expected to be charged with the sexual assault of two girls dating back to the 1980s.

Whitegoods repairman Bill Spedding was arrested by police on Wednesday and later questioned about the alleged assault of the girls, aged three and six, in 1987.

Homicide detectives arrested the 63-year-old at his semi-rural property at Bonny Hills, about seven months after William vanished from the yard of his grandmother's Kendall home wearing his favourite Spider-Man outfit.

Son Rodney Spedding and his wife Aimy told Fairfax Media they would fully support Mr Spedding after hearing the news of his arrest on Wednesday.

"These are fresh allegations and we don't believe them to be true," the pair said.

Mr Spedding yelled "No" when five detectives informed him he was under arrest before he agreed to go with them to Port Macquarie police station.

The local businessman has repeatedly denied any involvement in the toddler's disappearance and police have maintained he is not a suspect, but was a "person of interest" and only one line of inquiry they are pursuing.

Mr Spedding visited the home of William's grandmother four days before the boy's disappearance after a request to repair a washing machine.

A close friend, Colin, said Mr Spedding did not return to the grandmother's house with a spare part for the washing machine on the day William went missing because he could not get in contact with her.

Police dug up the front lawn and drained a septic on Mr Spedding's Wandoo Place property in January and have spoken with him a number of times.

Officers also seized a mattress and computer from his Laurieton office for forensic testing.

The head of the homicide investigation Gary Jubelin visited Mr Spedding's home on Monday and in March when police conducted a three-day bush search between a Lake Cathie and Bonny Hills.

Last week William's parents made a stomach-churning plea for anyone who knew anything about his disappearance to come forward.

The toddler's parents say they cannot live not knowing where he is or what has happened to him since he vanished on September 12 last year.

"We need to know where he is and we need to know what happened to him .... 'cause we can't live forever... like this," his mother said.  "His sister can't grow up never knowing what happened to her brother."

William's parents say they always suspected their three-year-old was abducted and had not simply wandered off from the yard where he was playing with his sister.

"If somebody has him and if he is alive ... I want him to be safe, I want him to be feeling loved and I want someone to be looking after him ... because to imagine that something else is going on ... we can't live a life like that," his mother said.

Anyone with information was urged to call Crime Stoppers on 1800 333 000