NSW Police has launched its annual campaign to raise awareness of the problems and effects of missing people.
For National Missing Persons Week, police plan to highlight one missing person each day this week.
Detective Superintendent James Johnson said Green Valley police were appealing for help to find a young woman who had been missing for five years.
Rista Chanthavixay, then 15 years old, was last seen in Mount Pritchard, about 4.15pm on March 24, 2009.
Miss Chanthavixay, who would now be 21, was reported missing after she did not attend school.
Police say that Miss Chanthavixay was spotted by a friend at Campbelltown station a month after her disappearance.
She is believed to have been heading to Melbourne.
Miss Chanthavixay is described as being of Asian appearance, 160 centimetres tall, of slim build, with black hair and brown eyes.
Detective Superintendent Johnson said police would never give up searching and the community's contact with police would save a lot of anguish.
"People go missing for a number of reasons," he said.
"These could relate to health-related issues or disputes within the family.
"There are some people who choose to distance themselves from family due to other reasons which we will never know — but in all circumstances I urge any missing person to at least make contact with their local police to let us know that they are safe and well."
National Missing Persons Week runs until Saturday, August 9.
Anyone who knows of Rista Chanthavixay's whereabouts or who has information that may assist investigators is urged to contact Crime Stoppers on 1800 333 000.