The news is good for wildlife near the Three Kings Islands, as investigations suggest an oil spill in the area is smaller than originally feared.

A fishing vessel alerted Maritime NZ on Monday after it found oil on fishing gear and sea birds while close to the islands, which are north of Cape Reinga.

A Maritime NZ spokesman said it was unclear whether the oil had come from a boat or a drilling operation.

But it appeared the spill was much smaller than originally thought, he said.

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The immediate focus was on getting reliable information about the size and impact of the spill, Maritime NZ's incident controller Bruce Barton said in a statement.

Poor weather had made it difficult to communicate with the vessel, and had cancelled a helicopter flight that was planned for today.

On Tuesday a Royal New Zealand Airforce C130 Hercules operating nearby on another task was able to fly over the area, he said.

"This flight reported some possible small amounts of oiling on the shores of one of the islands in the group."

Today the NZ Defence Force was to fly an aerial reconnaissance flight to take pictures of the area.