Maritime New Zealand has observed the first signs of structural weakness to the container ship Rena, grounded on a Bay of Plenty reef since last Wednesday.

During this morning's first aerial observation of the vessel, following the overnight storm, MNZ salvage unit manager Bruce Anderson saw deformation of Rena's hull.

Photos taken of Rena this morning.

The deformations are stress marks on hull panels caused by awkward load bearing of the vessel's weight.

He says early indications are that the stressed areas are the points on the hull adjacent to where the ship hangs off the rocks.

These stressed areas appear 'wibbly-wobbly” along where the ship's side should be smooth.

The extent of the stressing is unclear at this time and surveyors can only use photographs and scientific data sourced from off the ship to assess it as there is no-one onboard.

Until today, the ship, which has its bow firmly resting on the reef, was damaged, but its hull was managing to support its weight despite more than half of it being above water.

Mr Anderson says the stress is caused by the ship moving in the high winds and heavy 4m swell brought on by the overnight storm.

The ship is on a starboard side lean of 15-20 degrees.

The treacherous condition of the ship in the heavy weather is making it impossible for the salvage team to re-board and attempt to reduce the ship's weight and pump oil off it.

Containers are falling from the ship into the water and oil continues to leak.

Maritime New Zealand is advising people who see a container wash ashore they are to call 0800 OIL SPILL or contact police, who are responsible for handling them.