Teen duo charged over NSW kidnap, carjack

Two teenagers who allegedly carjacked a young driver in Sydney's west last year, bundling him into the boot of his car and going on a spending spree over a 13-hour period, have been charged.

Police say an 18-year-old man was flagged down by a distressed teenage girl as he drove through Penrith in the early hours of a September 2016 night, but as the 16-year-old girl got into his car so did a male youth.

It's alleged the male, aged 17, threatened the driver with a knife and forced him into the back seat.

Police will allege that for the next 13 hours the teenage duo used the driver's debit card to withdraw cash and make purchases, with the victim either locked in the boot or being held in the back seat of the car, until he was dumped several suburbs away.

The accused teenage boy was arrested and charged a month after the alleged incident while the girl was charged on Thursday.

They have been charged with robbery in company, detain for advantage in company, and aggravated carjacking, and are due to face a children's court on Friday.