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More than 100km of Bay of Plenty coastline could be coated in oil - with wide-ranging ecological effects - if the fragile container ship the Rena sinks, it was estimated yesterday.

The Rena was estimated to be carrying 1700 tonnes of fuel - or about 1,785,000 litres - when it hit the Astrolabe Reef last Tuesday.

At least 350 tonnes of oil has leaked from the stricken ship, and onshore winds yesterday deposited the pollution as far north as Matakana Island and as far as Little Waihi in the south - about 40km of coastline.

Experts fear that the full cargo of fuel could be released if the ship breaks apart and sinks.

A simulation of the oil slick's movement created by oceanographer Brett Beamsley showed tentacles of oil spreading several kilometres to the west of the ship today. That was expected to swing back eastwards tomorrow, heavily affecting Motiti Island, which is home to a large fur seal colony.

Ocean currents and wind are expected to push the oil further east tomorrow and Saturday, in the direction of Whakatane and Opotiki.

Tauranga City Council also confirmed that oil had seeped into the inner harbour.

Mr Beamsley said it was difficult to create an accurate model of where the oil would reach, but weather modelling indicated a strong shift to the east of the Bay of Plenty.

Marine scientists said the present amount of oil pollution was harmful, but manageable.

Oceanographer Simon Boxall said that if 350 tonnes of oil remained 12km offshore the damage to the marine environment would be short-term.

That would mean holidaymakers would be able to swim at the popular Mt Maunganui and Omanu beaches this summer.

On Tuesday, Environment Minister Nick Smith estimated that the oil currently in the water could reach 50km of Bay of Plenty coastline.

If the entire container ship's load was to leak into the ocean, unfavourable winds or currents could see its effects stretch much further.

Some experts feared the pollution could reach as far as Te Kaha, nearly 200km away.

Australian marine scientist Ian Poiner said it was difficult to predict the scale of damage if the ship sank, but he pointed to the example of a recent Queensland oil spill.

The container ship Pacific Adventurer leaked 250 tonnes of oil onto Sunshine Coast beaches in March 2009. That amount - smaller than the slick in the Bay of Plenty - coated 75km of coastline and required the removal of 3000 tonnes of oil-contaminated sand.

A biologist at James Cook University in Queensland, Dr Norm Duke, said the longer the oil remained floating at sea the safer it became. "And the rougher the weather, the better also."

Niwa marine scientist Drew Lohrer said that in large oil spills overseas, when the oil remained in a large slick, it had been "slurped" back into vessels or burned off. "So the rough weather breaking it up would make that impossible."

A Herald walk on the beach yesterday found a thin film of oil in the tidal zone from Mt Maunganui to the worst affected spot, Papamoa, with the wild dunes and shoreline pockmarked by cowpat-like piles of viscous oil.

While some communities on the outskirts of Tauranga had an anxious wait to see whether the oil would reach their shores, many were already decrying the interruption to their beachside lifestyle.

Resident Phil Josephs said he demanded that his boss give him a day off so he could begin clearing spots of oil from the sand.

"When you live by the coast, there's a way of life - you surf, my kids surf, we dive, we catch snapper, we eat tuatua from the sand, have a cold beer with mates in the dunes. But now they're talking about this stuff being here for two months. Maybe years."

Visibly upset by the sight of a streak of purplish oil in the water, he said: "I never, ever thought I would see this in New Zealand."