Vigorous seas are reducing the chance of tar balls washing ashore after a split pipe at an offshore platform caused 250 litres of oil to spill into the sea.
The slick has spread 10km from the Raroa platform site off the Taranaki coast since Friday.
The spill happened when oil company OMV was transferring oil from Raroa to the oil tanker Nectar.
Maritime New Zealand (MNZ) earlier said the oil could turn up as tar balls if it hit the coast. But after a flight over the spill area this morning, that now seemed unlikely.
MNZ spokeswoman Sophie Hazelhurst said there were no concerns for local wildlife.
"The environment's obviously doing a really good job of breaking it up," she said. "The conditions out there are quite dynamic."
She said "trajectory modelling" software indicated any oil was still a few days from reaching the shore, if it made landfall at all.
The spill happened about 70km from the coast.
It was very unlikely any beaches would have to be closed, Ms Hazelhurst said, and too early to say what, if any, punishment OMV would receive.
"The key point at this stage is looking at what the cause was and making sure it's rectified before they resume operations."
Jean Kahui of Frack Free Kapiti said her organisation was concerned about the "disgusting" spill and said it raised wider concerns about offshore oil operations.
"We can feel extremely lucky that this was a small spill. Imagine if it was a larger spill. Our coast [would] be devastated for generations," she said.
Frack Free Kapiti said the issuing of offshore drilling permits should stop, at least until "Kiwis have been consulted and a national policy statement" developed to cover the issue.
OMV said in a statement it "sincerely" regretted the spill and would conduct a full investigation with MNZ.
The company said a transfer hose spouted a leak close to a connection point on Nectar, so oil spilled onto the ship's deck, where most of the spill was contained.
OMV said it reported the spill to MNZ immediately and had taken "all necessary steps" to remedy the situation.
The company owned shares in the Maui and Pohokura gas field and the Maui pipeline.