Almost four tonnes of oil has spilled into New Zealand's harbours and oceans since the Rena disaster.

Since the Rena ran aground on Astrolabe Reef off Mt Maunganui in October, 2011, spilling 350 tonnes of heavy fuel oil, a further 363 spill-related incidents have been recorded, a figure a leading marine biologist describes as "very sobering".

While most of the spills have been relatively small - ranging from a few millilitres to several thousand litres - Professor Chris Battershill of Waikato University likened it to "death by a thousand cuts" and said there was need for heightened vigilance.

OIL SPILLS SINCE 2011

Documents obtained from Maritime New Zealand under the Official Information Act show most of the incidents were related to commercial fishing, offshore oil exploration and refuelling facilities. There were also many spills involving recreational boats.



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Authorities say better reporting - Maritime New Zealand has to be notified no matter how small the spill - may be the reason for the high numbers, but they also admit not every spill gets reported.



In all, 39,000 litres was reported spilled between October, 2011 and August, 2015, most of it marine diesel, which doesn't persist in the environment as long as heavy fuel oil but can still be damaging.

SUPPLIED The Rena breaks up on Astrolabe Reef. Hundreds of tonnes of heavy fuel oil was spilled.

Tauranga, the country's biggest port, was one of the worst-hit, with a number of incidents since Rena. In 2013 a German-owned container ship spilled 1000 litres of heavy fuel oil and in April this year 1500 litres of oil leaked from a Mobil pipeline during refuelling, washing up on beaches and lawns around Tauranga Harbour.

Battershill, who led research into long-term environmental recovery after the Rena disaster, said he was surprised by the data.

"The thing that jumps out at you is the very large number of incidents. The overall frequency of these events is something to be concerned about," he said.

In some ways, Battershill said, the environmental impact of continual smaller spills in harbours and estuaries was worse than a big spill at sea.

"As dramatic as Rena was, it's difficult now to find any effect of that on the beaches. However, small spills within harbour systems, if they get into mangrove areas and sea grass, they can have a much, much longer effect. If you can see it, you can clean it up, as was shown effectively with Rena."

Battershill said Tauranga was one of the most pristine harbours in the world and had yet to be affected by invasive pests such as Mediterranean fanworm as Auckland's harbours had.

MARK TAYLOR / FAIRFAX NZ Pollution prevention officer Adrian Heays discusses the fallout from Rena.

The environment was quite resilient, he said, but his biggest fear was that on top of oil run-off from roads and stormwater discharges, on-water spills would create the right conditions for invasive species to thrive.

Dead birds stricken by oil from the Rena. Photo: MIKE HUTCHINGS

"These continuous pollution events knock out the resilience of the harbour system. The reason marine invasives take off is because they are used to polluted, compromised environments - they do better than native species."

He said that although heavy fuel oil stayed in the environment for longer, marine diesel could also have a devastating impact on juvenile fish and invertebrates.

Oil residue washed up on Papamoa beach after Rena. ANDREW GORRIE

A "massive load" of pollutants was coming from the thousands of recreational boats which went out each weekend, and he called for more vigilance from all water users.

"It's just sloppy behaviour of boaties often...you see people being a bit careless about changing fuel lines over and things like that. And there's a need to make sure there are failsafe systems around the large fuel lines inside ports."

SMALL SPILLS, BIG IMPACT

Nigel Clifford, Maritime New Zealand's general manager of safety and response services, said there was no formal definition of what constituted a significant oil spill.

"It's more about the consequences, and they depend on a whole range of factors like where did the oil get spilled, what type of oil is it, what are the winds, tides and currents, what is the nature of the environment?"

He said the spill from the Mobil pipeline in Tauranga Harbour in April was a good example.

"It was actually a very small quantity of oil in a global sense, but it caused an enormous amount of consequences. It got into reed beds, close to shore, in a sensitive area in terms of environment and culture. If that same volume of oil spilled 150 miles offshore, it would possibly have far fewer consequences."

Clifford said spills were generally becoming fewer and smaller, and there was better reporting of them, although there was a blip three years ago.

"2012 was a bad year, there were nine [spills over 1000 litres]. We would see that as unusual."

Clean-up after an oil spill at Timaru port in 2013. Photo: NATASHA MARTIN

Fishing vessels were most frequently involved, and marine diesel was the most common fuel spilled.

"You can boom for [diesel] and contain it. It has quite a reasonable evaporation rate so a lot of it disappears."

Clifford said Maritime New Zealand was focusing its prevention efforts on the areas of greatest risk.

"We're never happy if there are any spills. In the last two or three years we've turned our attention to the fuel transfer sites.

"It varies enormously from a single pump on a wharf in the middle of nowhere to sophisticated bulk fuel transfer depots for large scale refuelling. We're doing a whole bunch of work around those...it's a point of vulnerability."

He said Maritime New Zealand had done a lot of work around its oil spill response capabilities since Rena and now had a better ability to respond to bigger spills.

"We're trying to have a cleverer system, we move away from volumes to more of a consideration of risk and what's appropriate given the circumstances."

OIL STILL WASHING UP

Six months after 1500 litres of heavy fuel oil spilled from a Mobil pipeline into Tauranga Harbour, globules of oil are still washing up in reeds along the edge of the estuary.

Mangatapu resident Martin Neill's jetty and retaining wall were covered in oil after the April incident, and today he points out what look like large cowpats on the sand at low tide.

Oil washes up at Mangatapu after a Mobil pipeline spill. SUNLIVE.CO.NZ

"There's still glugs there, I've just been gradually digging them out. This will go on for years, it's not a 12-month fix, with the marine life it effects."

Neill said the Bay of Plenty Regional Council didn't act quickly enough.

"When I phoned up and said 'there's a tonne of oil at me bloody gate' they had someone come out the following day, that's far too long. If they'd got booms around it straight away they could have saved hundreds of thousands of dollars."

Neill believes there should be a barge permanently tied up at the Port of Tauranga with all the best clean-up technology.

"I want them to put a proper spill programme in place. They say they have - I don't believe they have."

Adrian Heays, the council's maritime pollution prevention officer, said it wasn't a matter of "click your fingers and go" when oil washed up.

"It takes time to mount a safe response. You need to get people kitted up and decontamination and the rest of it. Local people want to charge in, but you can make it worse."

There were also cultural sensitivities because an urupa was nearby. "It was a king tide and the oil was thrown quite high up out of the water, it wasn't going anywhere in a hurry. It was better to set up properly, do it once and do it right."

Heays said the council's response capability had improved since Rena, but "that's not saying it's ideal, we've reviewed what we did and have taken on recommendations. The more resources I get the better, I can always use more, but there has to be a balance."

Mobil has so far paid $836,000 for the council's clean-up costs, and may face charges. More than 200 insurance claims have been lodged, mostly for cleaning oil-stained boats.

Bags of oily waste piled up on Mangatapu resident Martin Neill's lawn after a spill in April. Photo: BOP REGIONAL COUNCIL

Samantha Potts, Mobil's public affairs manager, said the leak occurred from two small corrosion holes in a pipeline. A pressure test had been carried out the previous month and found no issues, she said, and an investigation was under way into how the failure happened. The pipeline remained shut down.

Heays said there was no excuse for spills - "there should not be a single spill of heavy fuel oil in the harbour, full-stop" - but there was far greater awareness among industry players and the public since Rena.

"Rena was a hugely traumatic experience and has left a big mark, but ...I think there's a whole range of benefits and it's flowed across New Zealand.

"There's a real atmosphere of proactivism around preventing oil spills in New Zealand now and I think it's translating into action on the ground."

* View the Maritime New Zealand Official Information Act release in full