In the first, a person was taken to hospital after a truck rolled off a road near the city's Northern Motorway about 12.10pm.
A police spokesman said the crash happened on Steep Hill Rd, close to the motorway.
A St John spokeswoman said one person was taken to Dunedin Hospital with moderate injuries.
An Otago Daily Times photographer at the scene said the truck, which was carrying an excavator, appeared to have rolled on to its roof after going down a steep bank.
A little later, about 12.40pm, a person was seriously injured when a car rolled down a bank on the outskirts of Mosgiel.
A police spokesman said officers responded to a single-vehicle crash near the intersection of Saddle Hill and McMaster Rds.
Fire and Emergency New Zealand and St John also attended.
A St John spokeswoman said one person with serious injuries was taken to Dunedin Hospital.
