One year on: Parents continue search for William Tyrrell

William Tyrrell, 3, disappeared while playing hide and seek with his sister.

The mother of 3-year-old William Tyrrell told police in a Triple Zero call her son had been "roaring around the garden" wearing his Spiderman outfit just before he went missing without trace.

Police on Monday released audio of that call as they sought public help in identifying several vehicles parked near the boy's grandmother's house in the small NSW town of Kendall on September 12 last year.

For nearly a year, the parents of the missing child have relived that day over and over again.

In the audio an emergency operator called Simone takes the call to hear William's mother say: "Hi, my son is missing, he's three and a half".

She says her family had been looking for him up and down the street for around 20 minutes but had not found him.

"We heard him roaring around the garden and then I thought, 'Oh, I haven't heard him, I better go check on him."'

The mother describes her son to Simone and tells her what he was wearing.

"He's wearing a Spiderman outfit ... he's got dark sandy-coloured hair, it's short, he's got really big browny-green coloured eyes.

"He's got a freckle on the top of his head when you part the hair on the left-hand side."

Williams's mother says she had noticed no suspicious people in the neighbourhood and it was the first time her son had gone missing and it was out of character for him.

Investigators believe William, who was waiting for his dad to arrive back at the home, may have run down toward the road when a paedophile saw their opportunity.

Police want help in identifying two vehicles parked in the street, one described as a dark grey old model medium-sized sedan and the other as an old white station wagon.

- AAP