Weather reports over the Bay of Plenty, where the MV Rena is grounded on a reef, said the area will experience increasing winds on Thursday at speeds of 30 knots, bringing swells up to 4 metres high. Marine authorities fear damages the bad weather could inflict on the ship.

The bad weather is forecast to continue for the rest of the week, and will settle only after Saturday, the New Zealand Herald reported.

A 30 metre-wide rainbow oil sheen was yesterday seen stretching up to 5 km from the vessel, as some darker oil patches remained visible around the ill-fated vessel.

Meantime, salvors have been working on a crane used to lift containers from Rena to improve recovery efforts and vessel buoyancy. However, efforts to clear containers from the ship have been halted again in view of the bad weather.

Dive teams inspecting the Rena's hull reported slight changes in the buckling in the starboard side, but motion sensors attached to the hull have detected no significant change in movement, the Herald reported.

Meanwhile, most of the birds and wildlife rescued from the oil spill have recovered, with the rescue centre now looking as it did when it has just opened over two months ago to respond to the disaster.

About 50 more little blue penguins will be set free at Mount Maunganui beach Thursday morning, and this could be the last large- scale release following the marine disaster.

About 60 other penguins are set to remain at the rescue centre, either because of their health or because their homes are not yet clean.

An estimated 100 tonnes of oil is still believed to be stuck in Rena's ruptured belly.