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About 30 litres of oil was spilled into the Waitemata Harbour on Saturday from a cargo ship, the Mount Travers, which was carrying sugar from Hong Kong.
The duty officer at the harbourmaster's office, Mick Courtnell, towed sorbent booms from a boat to soak up the oil and it was sent to the Wiri Oil Terminal in Manukau for disposal.
He said an investigation was under way into how the spill occurred and the harbourmaster would interview the crewmen and check to see if they had a spill prevention plan in place.
They could be prosecuted or face a fine under the Resource Management Act or the Maritime Transport Act. The ship's master might also have to pay for the cleanup, which could be up to $5000.
