Police have 'serious concerns' for a teenager who has been missing from Sydney's south-west since Thursday, after she did not return home after school.

Sarah Ephraims, aged 15, was seen leaving a home before school on Newbridge Road in Moorebank, in Sydney's south-west, at 8am on Thursday.

Police were contacted when she failed to return home on Thursday evening, with investigations continuing throughout the weekend.

She was last seen wearing a white and maroon high school uniform, and is of Caucasian appearance, about 160cm tall, of slim build with brown hair and brown eyes.

Her family and authorities have 'serious concerns' for her welfare, police said on Sunday afternoon.

Investigators believe Sarah may have travelled to Wagga Wagga.

Police are urging anyone with information about Sarah's whereabouts to contact Crime Stoppers on 1800 333 000 or Liverpool Police Station.