An elderly couple have been allegedly assaulted and had their car stolen after being offered help with their grocery shopping by a stranger in Sydney's west.

A man is said to have asked for a lift after helping the couple with their shopping in Casula on December 13. NSW Police report the 81-year-old woman driving the car noticed the man allegedly steal her phone from her handbag before pulling over and asking him to get out.

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After leaving the gold Nissan Micra, he allegedy assaulted the woman and drove off with the 90-year-old passenger still in his seat before pushing him out in the nearby suburb, Liverpool.

"Emergency services attended, where NSW Ambulance paramedics treated the 90-year-old for facial injuries," NSW Police said in a statement.

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"He was taken to Liverpool hospital in a stable condition."

Police have released CCTV and images a man who may be able to assist with their enquiries.

The man is described as being of Caucasian appearance, about 50-years-old with short dark hair and light stubble.

"He was wearing a black, long sleeve jacket, grey tracksuit pants, black slides and a black cap," NSW Police said.

Police are urging anyone with information to contact Crimestoppers on 1800 333 000.