The oil slick from a leaking ship off the Tauranga coast remained a constant size today, Maritime New Zealand said.
But the environmental damage from the spill will get worse before it gets better, Transport Minister Steven Joyce said.
And the spill is taking its toll on wildlife, with four little blue penguins and two shags rescued from Motiti Island today.
Four teams of responders are based on the island at present, and 10 more will join them tomorrow.
The Rena has been stranded off the Tauranga coast since grounding early on Wednesday. It has 1700 tonnes (2 million litres) of heavy fuel oil on board.
Transport Minister Steven Joyce arrived in Tauranga today to inspect the leak, SunLive.co.nz reported.
"We are dealing with a considerable clean-up operation and we can expect some oil to come ashore at some point," Joyce said.
An estimated 100 tonnes of oil has leaked from one of the ship's fuel tanks into its bilges and keel and forecast bad weather is expected to increase the rate of spill as the ship is damaged further.
Maritime New Zealand National On Scene Commander Rob Service said efforts to stop the leak and disburse the oil were difficult due to inappropriate equipment for the rough seas.
"Offshore operations are subject to weather conditions and we are not being able to undertake on-water operations due to the conditions," he said.
More than 100 people are now in Maritime New Zealand's response team, which is planning for all eventualities, including a large-scale discharge of oil from the ship.
Trials of dispersants were continuing after inconclusive results yesterday, with experts advising against the use of protective booms due to strong currents and rough seas, Maritime NZ said.
The Auckland based fuel ship Awanuia will head to Tauranga shortly in what is likely to be a bid to get bunker oil off the stranded ship Rena.
The Department of Conservation (DOC) and National Oiled Wildlife Response Team members were monitoring the area for any injured wildlife and checking on reports from the public.
A specialist oiled wildlife response centre has been established at a water plant outside of Tauranga.
MNZ advised people to not handle any oiled animals found themselves, and reminded people there was a 1km exclusion zone in place.
Marine experts are warning it's highly like the ship would break up and say the fuel needs to be taken off.
The call came as Environment Minister Nick Smith said the spill from the ship "had the potential to be New Zealand's most significant maritime pollution disaster in decades".
Marine expert John Riding, of Marico Marine, said the ship's position tracking based on AIS data - a system which gives real-time ship locations - showed the Rena had moved slightly.
"I truly doubt that she will float if she breaks in two and unfortunately that is a real scenario, given that she is laden.
He said more trouble would come if sea swells built, which were forecast for next week.
Low water had revealed the true extent of problems facing the specialist salvage team tasked with solving the growing crisis.
From an observation boat the ship's bow could be seen rammed up and onto the reef, listing dangerously to one side. Containers stacked eight-high on the ship's deck looked to be compounding the problem.
Observation flights that began at first light, continued to survey the damage while specialist dive crews were now in the area and inspecting holes in the hull where the ship struck the reef.
It was through these holes that heavy fuel oil was leaking into the water, putting fish and other marine life at risk.
"Never in my 30 years of being on these waters have I ever seen anything like this," said marine expert Graeme Butler.
"This is my worst nightmare - how dare these people come to my ocean environment and do this to it."
GOVERNMENT 'SHOULD TAKE OVER'
Opposition parties were calling on the Government to take responsibility of the salvage operation.
Green MP Gareth Hughes said the Government should take over full legal responsibility for the clean up due to the seriousness of the environmental situation.
It was good the Government was cooperating with the vessel's owner, but the public needed to know where ultimate responsibility lay, he said.
"This situation needs the full resources of our Government to resolve this dangerous environmental situation."
Labour conservation spokeswoman Ruth Dyson said it appeared the Government had learnt nothing from the oil spill in the Gulf of Mexico.
"The Government should be all over this. It needs to take the lead and work alongside the salvage company, and it needs to ensure every effort is made to disperse the oil."
Wildlife experts said the oil leaking into the water was a disaster in an area teeming with marine life, she said.
"It's not something waiting to happen. It is happening."
Svitzer had experts onboard the ship examining how to best resolve the situation.
Spokesman Mathew Watson said a naval architect was arriving from Holland this afternoon to help with the assessment.
"He has a pivotal role because he calculates the situation of the vessel and the various weight scenarios."
Watson said it could take a couple of days for equipment from New Zealand, Sydney, and further abroad to land in Tauranga.
"That is quite normal for a salvage operation of this magnitude," he said.
The operation could take some time as it was a "steady and meticulous" process.
"It's not something that can happen overnight. There are hundreds of variables that need to be considered and the plan needs to be gradually developed."
"The priority at the moment is focusing on the fuel and how that can best be dealt with."
Watson could not say how the fuel would be drained, but said experts could move the fuel to another location within the vessel or transfer it to a bunker barge.
"In salvage operations such as this, there are different options available. With which we'll proceed with the Rena, it's too early to say."
A bird cleaning and rehabilitation centre has been set up in Tauranga today and a base was established on Motiti Island.
Forest and Bird seabird advocate Karen Baird said urgent action was need to avoid an environmental disaster and minimise seabird deaths.
The major risks will be to seabirds such as terns, gulls, gannets, penguins, petrels and shearwaters.
"Landing in the oil slick is a death sentence for these birds. Their feathers will become clogged with oil and they can sink or drown, or be unable to fly. Swallowing even small amounts of oil can be fatal to them or their chicks when they try to feed them."
- Paloma Migone, Kirsty Johnston/Stuff, Michael Field/Auckland Now, SunLive.co.nz