Senior Constable Richard Whitmore, of Clinton, said an Audi A6 towing a trailer containing two trail bikes was travelling north on State Highway 1 when it went out of control approaching a corner.
The car and trailer ran off the road, coming to rest in a ditch at Kaihiku, about 15km north of Clinton, shortly after 3pm.
The 50-year-old Balclutha man driving the car was uninjured but his 8-year-old son suffered an arm injury and was taken to Dunedin Hospital by St John.
Anyone who witnessed the crash is asked to contact the Clinton police on (03) 419-9060.
Emergency services were also called to a crash earlier that morning, at Ashley Downs near Clinton.
Snr Const Whitmore said an 18-year-old Wairuna man was driving a Toyota utility vehicle on Clydevale Rd towards Clinton when he lost control of the vehicle on a straight section of road.
It crossed the centre-line, crashed through a fence and rolled, coming to rest on its roof in a water-filled culvert.
The 18-year-old driver extracted himself from the vehicle and went to a nearby house for help.
He suffered neck and head injuries and was taken to Dunedin Hospital.
While emergency services arrived shortly after 7am, it was believed the crash happened about two hours earlier, Snr Const Whitmore said.
Inquiries into both crashes continue.
