Missing 16-year-old Sydney teen, Cassie Olsczac, has been found alive by NSW police.

Sydney teenager Cassie Olsczac has been found by police, five days after disappearing from the city’s south.

According to police, a motorist this morning spotted a person matching the girl’s description on the Princes Highway at Waterfall, and raised the alarm.

She was approached by police, who confirmed her identity. She is reportedly safe and well.

Missing since Sunday evening, the 16-year-old girl from Sylvania was last seen at approximately 6.30pm after getting off a southbound train at Banksia railway station.

She was supposed to be catching a connecting train to Hurstville, but it’s believe she never got on board.

The teen’s disappearance sparked a wide-scale land and air search by police and SES, which focused on the Royal National Park, south of Sydney.

CCTV footage captured Cassie's last known movements before she vanished.

Cassie's distraught mother had made several public appeals for information about her daughter's whereabouts, saying her disappearance was completely out of character.

Speaking yesterday to Mamamia, Connie Olsczac, said her daughter had an argument with friends at around 5pm on the day she went missing.

"She was upset. She may have wanted to leave because she was upset. She may have wanted to digest that information," she said.

"Perhaps other people are helping her at this stage. If they are, I’m begging you, she needs medical attention, you are not doing her any favours keeping her away from her family."

More to come.