The first dead birds have been found in oil which leaked from a container ship stranded off New Zealand, authorities said Thursday.

Maritime New Zealand (MNZ) said four dead birds had been discovered in the slick and two wildlife rescue centres had been set up.

MNZ has officially declared the 47,000 tonne container vessel "Rena" a hazardous ship after it ran aground on a reef early Wednesday about 12 nautical miles (22 kilometres) off the North Island.

The authority also said Thursday the oil slick had more than doubled in size in just a few hours.

MNZ on-site controller Rob Service said a monitoring flight over the stricken vessel on Thursday afternoon revealed the oil slick was five kilometres long, up from two kilometres earlier in the day.

He said oil was still leaking intermittently from the Liberian-flagged vessel, apparently coming from pipes damaged when it hit the Astrolabe Reef off the coast from Tauranga.

"We are not aware of any actual breaches in the fuel tanks. However, because of the extensive damage to the vessel, it is difficult to determine accurately what the scale and scope of the damage is," he said.

Service said it was not clear how much of the 1,700 tonnes of heavy fuel oil on board had leaked.

The Astrolabe Reef is covered with colourful sponges and anemones, according to the Department of Conservation, with seals and gamefish such as marlin common in surrounding waters.

Service said the MNZ response team was testing whether dispersants could be used to break up the slick.

"Reports are that it is going well," he said. "We will review the results from the trials later today, with a view to launching a full dispersant operation tomorrow morning."

MNZ said the declaration that the "Rena" was a hazardous ship gave the agency power to take control of the salvage operation if it was deemed necessary.

A salvage team appointed by the ship's owners was already on board, it added.

None of the ship's 25-man crew was injured when it ran aground. Why it stranded remains unknown and is subject to a Transport Accident Investigation Commission inquiry.