A large petrol/diesel spill is causing congestion and dangerous driving conditions on State Highway 1, north of Wellington, tonight.

Police said it was not clear what caused the spill near Pukerua Bay.

A number of cars have lost control on the slippery surface and at least two accidents had been reported.

No one has been hurt.

Emergency services and roading contractors were in attendance and motorists should expect delays in both directions while the spill was cleared.