The New South Wales Premier is urging people to be aware of the risks of meeting people online.

On Friday police found the body of 18-year-old Nona Belomesoff in Sydney's south west, two days after she went to meet a man she had befriended on Facebook.

The Premier Kristina Keneally, says it is a tragic reminder that people do not always represent themselves truthfully online.

"People may not be who they purport to be on social networking sites and that can be the case," she said.

"It is a reminder to us all no matter what our age that we should be careful of the people that we meet online."

Ms Belomesoff left her Cecils Hill home on Wednesday morning to meet two men she had befriended on Facebook.

Detective Russell Oxford says the teenager believed she was going on a trip to work with an animal welfare organisation.

"She was obviously excited to perhaps start work in an area with animals, which she loved," he said.

Detective Oxford says police have been shaken by the murder and that the scene was devastating.

"I've been doing this for a long time and we all got very upset with what we found last night," he said.

"We go out in the dark with torches and we find this young girl, 18 years old lying in a creek bed - it's just terrible."

On Friday night 20-year-old Chris Dannevig was charged with her murder.

He has been refused bail and will appear at Campbelltown Local Court in the next week.

Police say they will allege that the second man Ms Belomesoff believed she was meeting did not exist.