Sales of both new and used vehicles dropped in July, compared with the same month last year.
Last month, 5,429 new cars were sold, 953 fewer than in July last year. The seasonal fall in new vehicle sales was the biggest since the 1990s.
Combined sales of new cars and commercial vehicles fell 15.7% compared with July last year, while sales of used imported vehicles fell 36.8%.
Motor Industry Association chief executive Perry Kerr says it has been a roller-coaster winter. He says there were signs of a slowdown in new vehicle sales in May, but they bounced back strongly in June.
Mr Kerr says the figures for July reflect recent negative economic data.