The oil supplier Refining NZ says oil is spilling from a crude carrier discharging its cargo at the Marsden Point wharf in Whangarei Harbour.
Refining NZ could not say exactly how much oil had leaked, but said it was a small amount that was being contained around the vessel.
It said it had identified where the leak was coming from and would begin pumping the oil off the surface - a process that would take until the early hours of tomorrow morning.