The terrifying encounter on Sunday came three weeks after a 12-year-old girl was grabbed from behind while on her way to school, dragged into bushland and sexually assaulted.

Both victims were targeted in Narara and provided police with similar descriptions of their attacker.

The latest victim, an 18-year-old woman, told police she got off a train at Narara railway station about 4.30pm on Sunday when a man tried to abduct her.

She walked along Narara Valley Drive and into a laneway leading to Koninderie Parade when the man, who wore a grey hooded jumper, grabbed her around the throat.

"She managed to fight and kick that person and make an escape," Sex Crimes Squad Detective Acting Superintendent Mick Haddow said on Monday.

The woman ran away, screamed for help and called triple-0.

In similar circumstances on May 15, the 12-year-old was on her way to school at 7.45am in Narara when a man grabbed her from behind.

The girl told police the man, carrying a hunting knife, tied her up and sexually and indecently assaulted her.

It is understood the girl had injuries consistent with what she told police had happened.

The girl eventually escaped and ran to school where she raised the alarm at 9am.

Police say descriptions of the attacker in both cases were "strikingly similar".

Both victims have told police their attacker was in his mid-20s, at least 175 centimetres tall, with blond or ginger hair and blue eyes.

They also said he was wearing camouflage pants and had a loose covering over his face that exposed his eyes and nose.

"We are concerned about these matters, clearly, and we are seeking help from the public and people in that area ... who may know who is the owner of, or wears, the camouflage clothing I have described," Detective Haddow said.

It was concerning that the attacker might live in the area, he said, but he would not elaborate on any suspects police had in mind.

"It is concerning that a person would be prepared to wear camouflage clothing and cover their face in the way this person has," he said.

"It certainly says something about the criminality involved and is one of the things we are very concerned about."

Extra police patrols have been deployed to the Narara area with parents and children urged to be aware of their surroundings.

Detective Haddow said child abduction was very rare in NSW and cases like it relied heavily on information from the community.

At the time of the May 15 assault, Brisbane Water police Superintendent Danny Sullivan said the schoolgirl was on a common thoroughfare regularly used by students and dog walkers when she was grabbed.

He said the girl was understandably distressed, but was working closely with police and hospital staff.

"She was able to give us a very clear description of the alleged offender," Superintendent Sullivan said.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.

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