Sergeant Craig Brown, of Dunedin, said three vehicles lost control on the same section of road last Monday, one of them rolling into a ditch.
On Thursday night, a 57-year-old female driver with a 55-year-old passenger lost control on the wet, slippery road and also rolled into a ditch.
Police did not know whether the drivers failed to exercise due care in wet conditions, or if road design was a factor, Sgt Brown said.
The crashes all happened on corners several hundred metres above Substation Rd, where three crashes occurred in less than three hours on January 14. A motorist was taken to hospital after a crash on the corners in June.
In January, Dunedin City Council roading projects engineer Evan Matheson and senior contract supervisor Peter Hughes were unable to determine the cause of the slippery conditions. They said the most likely reason was a mixture of rubber and fuel residue from passing vehicles.
Mr Matheson on Friday said the cause of the recent crashes was "not immediately obvious".
"It could be a combination of factors other than the surface condition, which is wearing but far from worn out. It has us scratching our heads but is becoming quite a significant area of concern. The stats are unacceptably high."
Work on the corner, and possibly on a second corner 200m on towards Mosgiel, was needed quickly, he said.
The council did preliminary road-surface and geometric investigations this year and a road-safety audit was completed in July. While kerbing and warning-sign issues were highlighted, no obvious defects were identified, he said.
"But we need to review those findings."
All information would be passed to the DCC safety team. Additional warning signs would also be put in place.
The road was last sealed in 2004 and was treated for "flushing" in 2006. Flushing, where bitumen diluents such as diesel and kerosene seep through to the road surface, was not the cause of the slippery conditions, he said.
The section of road is near a 2.6km stretch, between Substation Rd and Flagstaff Creek culvert, which reopened in November 2007 after a $1.15 million, three-month upgrade.
Three Mile Hill Rd is used by up to 4000 vehicles each day. Between 2001 and 2005, 37 accidents were reported on the road.
• A 50-year-old woman driver was taken to Dunedin Hospital after an accident at the bottom of Three Mile Hill Rd about 4pm on Friday. Constable Jonathan Wagstaff, of Mosgiel, said her car rear-ended another vehicle.
