Rough weather and strong swells around the Astrolabe Reef have resulted in a small release of weathered oil previously trapped under Rena.

Rena suffered significant damage to its hull when it grounded on the reef, and oil has intermittently leaked from the duct keel, which is a system of pipes running along the bottom of the ship, since the grounding.

Maritime NZ National On Scene Commander Mick Courtnell says salvors working on Rena yesterday saw blobs of oil floating from the wreck.

"We followed this up with two observation flights and our team estimates a small amount - perhaps half a tonne - of weathered oil has been released, probably from the duct keel.

"This oil has probably been trapped inside the wreck and exposed to sea water for some time.

Mick says it was possible some of this weathered oil would reach beaches at Papamoa and Mount Maunganui, and oil spill response teams would be on standby to clean it up over the next few days.

Affected areas of the beach may be cordoned off to allow oil spill response teams to work unimpeded if necessary. Mick asked members of the public to please be patient while this work was underway.

The forecast was for the rough weather 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.

"With the continuing swells we may see more of this oil come out of Rena," says Mick.

"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."

MNZ Salvage Unit Manager Arthur Jobard says the strong swells out at the reef were continuing to prevent container removal operations.

The swells were also putting stress on the damaged wreck, which remained in a precarious state.

"The electronic sensors used by Svitzer to monitor the wreck are not indicating any significant change in its movement.

"But these kinds of swells can cause more damage, and this is something we are watching very closely."

Both Svitzer, and Braemar Howells, the company contracted to recover containers washed overboard, have contingency plans in place ready to put into action if the condition of Rena deteriorates quickly.