Senior Sergeant Blair Dalton, of Dunedin, said the incident happened about 9am, when the driver was believed to have had a "medical event".
He said the vehicle collided with a number of road signs before forcing 20 competitors in the marathon to scatter for safety.
While this had the potential to be a serious incident, none of the competitors were hurt, Snr Sgt Dalton said.
He praised members of the public who came to the aid of the driver.
"Understandably, Police have had calls from people who were close to being run over, wanting to know what happened, and asking whether the driver was okay.
"The driver was assessed by St John and taken to Dunedin Hospital in a moderate condition for treatment.
"Initial indications suggest this was a medical event, and it’s incredibly fortunate nobody was hurt as a result.
"On behalf of Police, we want to say thank you to the people who went to the woman’s aid after she crashed and to those who called for help."
