Terrified residents in Sydney's northwest said yesterday their once carefree existence has been shattered with news a child sex attacker is on the loose.

The head of the NSW Child Protection and Sex Crimes Squad yesterday warned parents to be extra vigilient around their children after two eight-year-old girls were assaulted in Castle Hill.

"It seems to be very opportunistic," Detective Superintendent Helen Begg said.

"I would remind parents to be really vigilant about where their children are at all times...especially in parks and public spaces."

"If possible, try and get them to maintain a line of sight with you, so that you know where they are."

The two young friends were attacked as they watched their siblings play basketball in the sprawling Fred Caterson Reserve at Castle Hill on August 30.

Supt Begg said the girls' parents and police were only informed of the attack on Wednesday, when the frightened youngsters told their friends at primary school.

"The girls were very frightened during the attack and he did threaten them," she said.

"They were very, very frightened by what had happened.

The two friends were grabbed from behind by a

man they described as being aged between 35 to 50, tall, of slim build and with a pointed chin and "wispy" fringe.

Police revealed the man dragged both girls into bushes before sexually assaulting them.

Terrified residents said yesterday the attack shocked them as they had always considered their neighbourhood "fairly safe".

Mother-of-three Sue Cowie, whose street backs onto the reserve where the attack occured, said she often left the front door open and the keys in her car.

"But I'm going to have to think twice before letting the kids go to the park again," she told The Daily Telegraph.

Another resident said while news of the attack has shocked him, the fact the park is unpatrolled meant such an attack was "bound to happen".

"It is an unpatrolled carpark, and the road (into the reserve) is always open," the man said.

Strikeforce Soward, comprising of local police and officers from the Child Protection and Sex Crimes Squad to head the investigation, has been formed to investigate the attack.

Detectives spent yesterday and Thursday interviewing residents bordering the reserve on Gilbert Rd.

Police have also called on people who were in the park at the time to contact them with any information they may have.

Anyone with information should call Crime Stoppers on 1800 333 000.