What seemed like kindness from a good Samaritan quickly turned into fear and dismay for an elderly Sydney couple, who were carjacked and assaulted.

NSW Police arrested a 53-year-old man on Friday over the incident, which allegedly happened in Sydney’s south-west on December 13.

About midday, the Wyong man helped the couple with their groceries at a Casula shopping centre before asking for a lift.

After letting him into their gold Nissan Micra and driving off, the 81-year-old woman behind the wheel spotted the man swiping her phone from her handbag.

She pulled over and asked the man to leave.

He left the car but then allegedly assaulted the woman, got into the driver’s seat and drove off with the 90-year-old man still in the car.

The man is accused of assaulting the elderly man, pushing him out of the vehicle in Liverpool, and driving off.

After a police investigation, the man was arrested at about 8am on Friday and taken to Wyong Police Station.

Charges have yet to be laid.

-AAP