Two vessels with specialist oil recovery equipment and personnel Sunday began cleaning up oil spilled from a Liberian-flagged cargo ship grounded on a reef off New Zealand's North Island as severe weather was forecast to hit the area.

An oil slick streams from the Rena, a 47,000 tonne container ship grounded on a reef in New Zealand's Bay of Plenty. [Agencies]

Maritime New Zealand (MNZ), the country's shipping authority, said Sunday that two of the four New Zealand navy vessels in the area were also testing oil recovery equipment for use around the Astrolabe Reef, about 12 nautical miles offshore, near Tauranga, where the 47,000-tonne Rena, has been grounded since early Wednesday.

Preparations are also underway to transfer heavy fuel oil from the ship to the Awanuia, a double-hulled bunker barge, capable of holding 3,000 tonnes of oil.

MNZ national on-scene commander Rob Service said it would take a matter of hours to prepare for the direct transfer of oil, but several factors would determine exactly when that process would begin, under the direction of salvage firm Svitzer Salvage.

No further oil had been seen leaking from the vessel since Saturday and the oil spilled then had been dispersed, said Service.

"The removal of fuel from the ship remains the top priority. Pollutants on board the vessel including paint, grease, hydraulic oils and lubricants are being hand carried off the vessel on to a small support vessel," said Service.

"These are maintenance goods that are carried on most vessels. This is a labor-intensive but necessary contingency task."

Two ocean-going barges -- the Northern Quest and the Phoenix -- were tasked with recovering heavy fuel oil in the water and 10 specialist personnel from Australia were assisting.

Service said most of the Australian responders had worked on the Montara oil spill response in the Timor Sea in 2009.

"In terms of the oil recovery side of this response, there are similarities between the two operations. We have both New Zealand and Australian responders who were involved in the Montara response, so that experience is feeding in to how we manage the on- water operations," said Service.

"Our operations teams have done well to pull a lot of equipment for offshore use together very quickly. Our ongoing monitoring of the slick and its position allows us to target the operation to the priority areas."

Assessments were also being made of all the areas that could be affected by oil blown on-shore, said Service.

The New Zealand Defence Force had personnel on standby to move to Tauranga at short notice to help with a clean-up if it were required.

The oiled wildlife response was continuing with six field teams carrying out wildlife surveys on nearby islands and oiled wildlife patrols monitoring mainland beaches.

Another oiled little blue penguin had been taken to a wildlife facility, bringing the total number of oiled birds recovered to eight. All were in good condition and being washed and cared for, he said.

MNZ spokesperson Ross Henderson said MNZ was concerned about a severe weather warning issued for the area, with gale force winds and heavy rain forecast for Monday.

Prime Minister John Key told Radio New Zealand that serious questions needed to be answered as to why the Rena ran on to the well documented reef.

Speaking as the political storm over the accident and the government's response continued, Key said after flying over the stranded vessel that two inquiries were underway to get the answers, but the government was very keen to understand how such a catastrophe could happen.

Key said international experts had advised him that the operation to remove the fuel oil from the grounded vessel could begin Sunday night and could take 30 to 40 hours if it went well.

The Environmental Defence Society (EDS) Sunday described the grounding as "a very serious situation, made worse by the passing of every day."

"No attempts have been made to contain the oil with booms. There seems to be an experiment with dispersal agents. All the equipment required is not available in New Zealand and is being brought from overseas. Expertise also seems not available here and is being brought in and no containers have yet been removed," said EDS chairman Gary Taylor.

"And all this with worsening weather imminent. Winds are shifting on-shore and strengthening with heavier swells likely to increase the probability of stressing a vulnerable hull and the ship breaking up," said Taylor.

"It is extremely disconcerting to see how delays at every turn are increasing the risks. We would have thought that clear plans would be in place to deal with all conceivable incidents and that their deployment would be rapid and certain."

Conservation spokesperson for the main opposition Labour Party Ruth Dyson said that after four days of good weather since the ship grounded, the window to help had "pretty much closed."

"Tauranga is a major shipping port, yet much of the specialized equipment needed to deal with oil spills is having to be brought from Australia, and the experts from Holland," said Dyson.

"In the meantime we have marine and bird life suffering and irreversible damage being done to the environment."

The Green Party Sunday called on the government to put in place precautionary oil booms around sensitive coastal eco-systems to lessen the risk of large amounts of oil coming onshore.

"It is good to finally see an elevated sense of urgency from the government and Maritime New Zealand in response to the Rena's grounding, but we're still behind when it comes to coastal protection, should a catastrophic spill occur," Green Party marine spokesperson Gareth Hughes said.

Greece-based Costamare Inc., the parent company of Daina Shipping Co., registered owner of the 3,032-TEU Rena, issued a statement Saturday, saying it was cooperating fully with the New Zealand authorities and every effort was being made to control and minimize the environmental consequences of the grounding.

It said the Rena, which was built in 1990 and had a deadweight tonnage of 47,230, was carrying 1,351 containers when it grounded.

The 236-meter Rena was reportedly carrying milk powder, timber, meat, fish and a small amount of the hazardous material ferrosilicon, an alloy of iron and silicon used in making cast iron and steel, when it ran aground.