The Rena is spilling more oil and containers into the sea after heavy weather hit the battered cargo ship overnight, moving its stern.
The ship snapped in half on January 9, with its bow now sunk below the water line.
National on scene commander Rob Service said a light sheen of oil was stretching just under 1 nautical mile in an easterly direction from the wreck, and dark patches of oil have also been spotted nearby.
Service said while the amount of oil seen coming from the wreck was small, there was the potential for some oil to come ashore.
Small droplets of oil have been spotted at the high tide mark between Leisure Island and Tay St at Mt Maunganui beach. Service said these could be the result of the fresh release, or old oil being exposed due to the stormy weather.
"We'll be keeping a close eye on the movement of the sheen and our response team is ready for any fresh oil that reaches the shoreline," Service said.
Maritime New Zealand Tauranga response and recovery manager David Billington said a small number of containers have gone into the sea.
"This sort of movement and deterioration was not unexpected, given the rough weather conditions and that fact that the internal structure of the ship has been exposed to the elements for a long period," he said.
Billington said container and debris recovery company Braemar Howells has already collected one container containing milk powder by tug boat, north of Motiti Island, early this afternoon.
MetService forecaster John Law said conditions off the coast of Tauranga last night were rough.
"It was pretty unsteady in the area, with a continual wind speed of 47 knots (about 87 kmh), with gusts of up to 60 knots (111 kmh)."
MNZ said swells up to six metres hit the ship.
- With Fairfax