Julie Clarke, a child restraint technician from Baby on the Move, said the Safekids Coalition, which involved Baby on the Move, Plunket, St John, the Dunedin City Council and the police, held its campaign at least twice a year, in which it checked up to 300 cars over five days.
What technicians had encountered this year in Dunedin was "absolutely shocking" and the worst results in the four years they had run the campaign, she said.
Many restraints were out of date or unsafely installed. On others, seat belts and harnesses were loose and, in several, the restraint was not tethered to the seat.
In one car, two 3-year-olds were not in restraints, and in another a mother sat in the back with a 3-month-old baby in a frontpack and a seat belt around them both.
In one car, three car seats were expired and it was clear many other people did not realise the restraints had a use-by date.
Ms Clarke blamed the problems on a combination of things, including the seats being installed wrongly, and a trend towards people buying cheap restraints off the internet or from stores that did not fit them.
If people were unsure, they could take their cars to Baby on the Move or Plunket, where a trained technician would install the seat properly.