Rough weather and strong swells around the Astrolabe Reef have caused oil previously trapped under Rena to leak.
The ship suffered significant damage to its hull when it grounded on the reef, and Maritime New Zealand says since then oil has intermittently leaked from the duct keel - a system of pipes along the bottom of the ship.
National On Scene Commander Mick Courtnell said salvors working on Rena yesterday saw blobs of oil floating from the wreck.
Two observation flights were carried out and and the team estimated about half a tonne of oil has been released, caused by the rough weather.
"This oil has probably been trapped inside the wreck for some time.
"With the continuing swells we may see more of this oil come out of Rena, he said.
Rough weather was expected to continue today, peaking with 30-35 knot winds and up to 3m swells in the early hours of tomorrow morning.
The weather is forecast to settle quickly after that, said Mr Courtnell.
It was possible some of the leaked oil could reach beaches at Papamoa and Mount Maunganui, and oil spill response teams were on standby to clean it up over the next few days, he said.
Affected areas of the beach may be cordoned off to allow oil spill response teams to work if necessary and members of the public were asked to be patient while this work was underway.
"It's obviously frustrating, particularly to our beach clean-up teams and volunteers who have put so much work into getting these beaches to the state where they can be used by the public.
"But we have always advised that more oil would continue to come ashore and we remain ready to respond to whatever Rena throws at us.''
The strong swells at the reef were continuing to prevent container removal operations and were putting stress on the damaged wreck, which was still in a precarious position, said MNZ salvage unit manager Arthur Jobard.
The electronic sensors used by Svitzer to monitor the wreck are not indicating any significant change in its movement, Mr Jobard said.
"But these kinds of swells can cause more damage, and this is something we are watching very closely.''
Crane barge Smit Borneo is due to arrive in Tauranga from Singapore at 8am tomorrow , to assist with the container removal operation.