Police are working to track down a man who fled the park in Guildford after the attack on Thursday afternoon.

Police searched the park and surrounding area with their dog unit and Polair but were unable to find the man.

Detectives from the child abuse squad have formed Strike Force Sandyman to investigate.

Detective Acting Superintendent Peter Yeomans, who heads the squad, said on Friday that police were following a number of leads, including that the attacker was a convicted sex offender.

‘‘That’s one of our lines of inquiries,’’ Mr Yeomans told reporters at Police Headquarters at Parramatta.

He said the older girl was accompanying her sister when the predator entered and attacked them.

It was difficult to say how long the attack lasted, but an ‘‘incident of this nature ... would take a long time, in your own mind’’, Supt Yeomans said.

He said there were a number of people in the park at the time and police were examining CCTV footage from nearby businesses.

Mr Yeomans said police were also following up with a woman who told media outlets she saw a suspicious man leave the same toilet block in October last year. 

He described the attack as a ‘‘very unusual offence’’ and said the girls’ parents didn’t deserve any blame.  

‘‘They’re very good parents ... It’s the middle of the day, it’s school holiday time there’s plenty of people around,’’ he said.‘‘It’s a very harrowing experience for the family ... They are very, very deeply stressed by what has occurred to them and to their children.’’  

He said police were looking for a man described as having black hair, a thin build and wearing a black jacket with a white stripe down the side, and blue shorts.  

Police are appealing for anyone with information to contact Crime Stoppers on 1800 333 000.

AAP