Waikato Regional Council now says an oil spill in King Country did affect birdlife.
About 20,000 litres went into the Awakino River this week after a tanker crashed and rolled on State Highway 3 in the Awakino Gorge.
Most of the oil has gone out to sea.
The council said on Thursday that initial checks showed there had been very little impact on the environment.
But it now says up to 20 shags, terns and ducks got oil on them.
The council says Massey University and the Department of Conservation were in the area on Friday to help with the wildlife response.