Maritime New Zealand's morning observation flight of Rena has revealed the overnight storm did not cause further aerially visible damage to the stranded vessel.

Onboard this morning's aerial observation flight of Rena. Photo: MNZ

Oil spill response teams and salvors were on heightened alert overnight after forecasts of 35 knot wind and 5m sea state threatened to cause further deterioration of the ship's hull.

The hull is cracked and buckled in several places after spending nearly a month grinding on the rocks of the Astrolabe Reef, about 25km from Tauranga.

MNZ salvage unit manager Bruce Anderson says no additional containers have been lost overboard in the overnight storm.

'There is a light sheen of oil from the vessel, but this is consistent with what we have seen on previous flights and is likely to be further seepage from the duct keel,” says Bruce.

Weather conditions have eased this morning and the current conditions of moderate to strong winds and swells are forecast to remain for the next few days.

Bruce says the salvage team is ready to board the 236m container ship stranded on the Astrolabe Reef when conditions allow.

The bleakness of the Rena disaster was well reflected in this morning's murky and high swell conditions at the Astrolabe Reef. Photo: MNZ

MNZ national on scene commander Mick Courtnell says the oil spill response team is ready to respond to any oil that comes ashore.

There is 358 tonnes of heavy fuel oil still aboard Rena in the starboard five tank, but this is the only heavy oil still aboard with salvors having pumped 1070 tonnes off.

He says hundreds of people are available for a large-scale clean-up effort should that be required.

'We had shoreline clean-up assessment teams out at first light, assessing the beaches where oil could come ashore.

"They have reported very small amounts of fresh oil along the beach between Omanu and Papamoa.

'We have New Zealand Defence Force personnel and volunteers ready to respond if a significant leak occurs.”

Mick says trajectory modelling is being done at regular intervals to monitor where a fresh release of oil would go.

'The weather conditions are dynamic, and they affect the projected movement of oil.

'We are monitoring this closely, so we will know where to direct our resources if more oil is released.

'Rena has been grounded for four weeks now and during this time we have gathered a wealth of information and local knowledge.

'We have the planning, people and equipment in place to deal with this situation.”

Mick says wildlife response teams are in the field looking for affected wildlife.

A total of 406 birds are currently being cared for at the wildlife facility, including 60 New Zealand dotterel, pre-emptively caught to protect the local population.

The facility currently has the capacity to care for up to around 700 birds.