The 65-year-old male driver and sole occupant was not injured in the crash, which happened at 3.15pm.
Sergeant Tony Woodbridge, of Oamaru, said the man was driving south when his vehicle drifted right and hit the island opposite the gated entrance to the Oamaru Public Gardens.
Fatigue was believed to have been a factor, he said.
The road was not blocked and inquiries are continuing.
Sgt Woodbridge asked anyone who witnessed the crash to contact the Oamaru station on 433 1400.