A police officer has had part of his leg amputated after he was struck by a driver who ploughed through a random breath test site in south-west Sydney on Friday night.

A 22-year-old man has been since been charged over the incident, for alleged dangerous driving while using his mobile phone.

The senior constable was one of two officers seriously injured in the incident.

The highway patrol officers were setting up the breath test site on Campbelltown Road at Leumeah when they were struck about 8.20pm.