Access to the Splore Festival along the Clevedon-Kawakawa Road is blocked after a car crashed and toppled a power pole.
Powerlines have also fallen across the road and blocked it to traffic.
Police said there were no serious injuries in the crash, but it would be a couple of hours before the road was cleared.
Festival goers will need to travel via Miranda, south of the festival, until the road is opened again around 1pm.
There will also be significant delays for those heading to the NZ Polo open in Papakura as a result of heavy traffic in the area.
Police urged motorists to allow for extra time, "drive with patience and slow down".