Fresh oil is leaking this morning from fuel tanks on the container ship Rena that Wednesday ran aground on Astrolabe reef about 11.5 nautical miles offshore of the north end of North Island.

The cargo vessel Rena aground on Astrolabe Reef [Bay of Plenty Regional Council, New Zealand]

Maritime New Zealand, the agency in charge of incident response, says personnel aboard an overflight at first light today estimate that between 130 and 350 tonnes of fuel oil has spilled from the damaged ship.

The ship was heading towards the Port of Tauranga from Napier, carrying about 1,700 metric tonnes of fuel when she ran aground. An investigation into the incident is ongoing.

The oil spill is now New Zealand's biggest maritime environmental disaster and the clean-up could take weeks, government officials said today.

Over the past 24 hours, there has been more damage to the front part in the vessel and additional flooding in the forward holds, as the ship has settled onto the reef and leveled out somewhat.

The weather in the area of the ship is poor, with three to four meter (10 to 13 foot) sea swells and winds of 20-25 knots (37-46 kph).

Maritime New Zealand?Tuesday issued a precautionary navigational warning that containers may come off the stricken ship and cause a hazard to passing vessels.

At this stage, there have been no reports of lost containers, but the agency says given the severe weather conditions and the vessel's movement it remains a possibility, adding that containers may wash ashore.

The top priority remains getting oil pumped off the 32-year-old, 236-metre Liberian-flagged vessel, which has to be done before the ship can be salvaged.

Briefing media Monday, Prime Minister John Key said removing the oil from the vessel is "a complex operation and could take some time."

While Transport Minister Steven Joyce warned that the ship could break apart on the reef, MNZ says currently "there are no obvious signs of deformation of vessel."

All personnel have now been evacuated from the stricken vessel as a precautionary measure due to adverse weather and sea conditions. The vessel earlier called a mayday as precaution to expedite the safe removal of the remaining crew.

Dispersants are being tested on the newly spilled oil, but the swells and winds are making it difficult for the chemicals to do their job of breaking up the oil, MNZ officials say. The Corexit dispersant is 10 times less toxic than dishwashing liquid and the ingredients that make up the dispersant are in most shampoos, the agency says.