Most of an oil spill at Northport in Whangarei Harbour is now contained, including some that washed up at nearby Marsden Cove.
Regional harbourmaster Jim Lyle said the port reported the spill - estimated to be several hundred litres - at 8.30am.
It is thought to have come from a ship berthed at Northport near Marsden Point.
Specialised oil containment booms are now in place around the vessel and nearby beaches, and an oil skimming barge has been used to scoop up some of the spill.
Mr Lyle said high tide at midday left patches of oil along a several hundred metre stretch of shoreline near Marsden Cove, and small amounts of oil were found at the entrance to the cove's marina.
The shoreline should be cleaned up before the next high tide overnight and attempts were continuing to recover oil from the water, he said.
There have been no confirmed reports of oiled wildlife.
Mr Lyle said it was too early to say if any action would be taken against the ship's owners.