Police said the woman was badly shaken up which could have had serious implications for her pregnancy.
She was assessed in hospital and, fortunately, there were no issues.
The black car fled across the median and travelled south on Ensors Road.
The attacker who had been hitting the car got into an unknown vehicle and travelled north onto Aldwins Road, ahead of a cream or white flat-deck truck.
Police would like to hear from the driver of the black car and anyone else who can identify the attacker or his vehicle.
Businesses and individuals in the area with CCTV are asked to check for any evidence of the truck and the vehicle ahead of it on Aldwins Road.
Any information relating to this incident should be forwarded to Christchurch Police on 03 363 740, quoting file number 170719/0078.
Information may also be provided anonymously to Crimestoppers on 0800 555 111.