A six-year-old girl was trying to help her two-year-old sister to the toilet during a family picnic at a Guildford park when a man followed the girls into the public toilet block and sexually assaulted them, police say.
Detectives from the Child Abuse Squad are scouring footage from CCTV cameras at businesses and homes surrounding Campbell Hill Pioneer Reserve to try to identify the predator who attacked the sisters about 1.30pm on Thursday.
They also are looking at convicted sex offenders who may have been in the area at the time.
Acting Superintendent Peter Yeomans, the commander of the child abuse squad, said the girls' parents were a short distance away when the pair walked into the public toilet block together.
The older girl was trying to help her younger sister to the bathroom, he said.
A man with black hair, an average-to-thin build and wearing a black jacket and blue shorts followed them inside.
Acting Superintendent Yeomans said the man indecently assaulted the younger girl and sexually assaulted her sister in aggravated circumstances.
Both girls were crying from the "harrowing experience", Acting Superintendent Yeomans said.
"They (the family) are very, very deeply stressed by what has occurred to them and to their children," Acting Superintendent Yeomans said.
The girls have been examined at Westmead Children's Hospital and are receiving counselling, while the older sister has been interviewed by police.
She told detectives that the assault took a long time, Acting Superintendent Yeomans said.
"Their concept of time is quite different to probably an adult's concept of time. I would say it could have taken a few minutes, but that would be more guessing than anything. But to a child, how they explained it to the investigators last night, it took a considerable amount of time," he said.
"The little six-year-old is still very upset by what occurred. The little two-year-old, when it did occur, was very upset, was crying."
He said the girls' parents and extended family were close by, and should not be blamed for what had happened.
"I'd like to say right up front that we shouldn't put any sort of blame or any sort of inference as to what family or the parents of anyone did in these circumstances," he said.
"They're very good parents. It's the middle of the day, it's school holidays, there's plenty of people around."
Acting Superintendent Yeomans said it was a very unusual offence, and police were using all the necessary resources to find the offender.
He understood if there was angst in the community.
"We're doing our best to try to bring this matter to a head as soon as we can. There are a number of leads that we are following," he said.