The historic one-lane Kopu Bridge, near Thames, reopened at about 7pm, the New Zealand Transport Agency (NZTA) says.
The bridge - a key link on State Highway 25 between Auckland and the Coromandel Peninsula - was closed at 1.45pm after contractors working on the new Kopu Bridge noticed a fault with the swing span section of the old bridge.
However, the bridge was found to be structurally sound, NZTA spokeswoman Michelle Te Wharau said.
"The fault detected today was caused by a bolt failure on a steel plate which bridges a 100mm gap between the swing span section of the bridge and the main deck,'' she said.
A new plate has been welded into place.
