The Rena soon after its grounding on Astrolable Reef.

The 2011 grounding of the container ship Rena on Astrolabe Reef near Tauranga brought about one of the world's most tricky and costly wreck recoveries ever, a science conference in Nelson has heard.

But it wrought very little long-term environmental damage, scientists reported.

A string of researchers gave presentatations on aspects of the Rena disaster to the Society of Environmental Toxicology and Chemistry's Australasia conference at the Rutherford Hotel. They concluded the area had a lucky escape, partly because of the huge public involvement in the clean-up.

Lead presenter, Waikato University professor Chris Battershill, who holds the inaugural Bay of Plenty Regional Council chair in coastal science, said 8000 volunteers collected 1000 tonnes of oily waste from the Bay of Plenty coastline, "at the time the largest volunteer army every deployed" in an environmental clean-up.

He said the 236 metre container ship was carrying 1650 tonnes of fuel oil and 1368 containers, including 121 packed with perishables and 32 with dangerous goods when it hit the reef while taking a shortcut to the port.

The debris field extended to Great Barrier Island north of Auckland and to East Cape.

Battershill said 259 containers were unrecovered, including 33 that were on the seabed, reasonably intact.

"The remainder are suffering very serious destruction and degradation. They're in the debris field and the aft section of the ship."

About two-thirds of the ship's hull remained on the seabed, he said.

"We think it's one of the most complex wreck and recovery operations internationally."

Container ships did sink but often they broke their backs in high seas and very deep waters and were "lost to the world", or beached so they were easily accessible.

"This ship however sank in a tricky situation, a highly-exposed environment but still relatively close to shore."

There were "very small windows of workable time" due to changing weather conditions, the water was relatively deep beside the reef and now the hull was resting in areas near the limits of operational diving.

"It's complex in the sense that we're combining issues with the environment, economic issues and, importantly, cultural issues all at the same time. We have an integrated response and now of course in addition to all that, it's very expensive."

The salvage operation was already the second most expensive known globally.

"If the full hull is removed in its entirety it will over-reach the Costa [Concordia] in cost," Battershill said.

In 2013 the cost of salvaging that Italian cruise liner was reported as $US799 million, excluding repairing the damage to the island it struck in 2012.

Last year the Rena's owners commissioned a report which concluded full removal of the wreck would cost between US$425m and US$546m on top of the US$300m already spent.

Battershill said after the Rena started losing oil and cargo the public demanded answers, particularly around the dangers of the oil spill to the environment, fish and human health.

"Based on the literature available, we found it very difficult to answer these sorts of inquiries."

There wasn't much information on the toxicity of fuel oils or mixtures of toxic chemicals and oils or even the toxicity of the dispersant Corexit used to break up the oil slicks, he said.

But there were a number of long-term scientific studies monitoring the ocean environment in the area and these, along with some rapid studies to "backfill" parts that weren't covered, established benchmarks ahead of the pollution and debris spreading.

There were also oceanographic studies that tallied with the knowledge of local kaumatua to very accurately predict the spread of oil along the coastline.

"So we were able to identify hot-spots and heavy loading of oil prior to it actually happening and we were able to deploy effort."

Fifteen scientific projects were then designed and linked to the Environment Ministry's long-term Rena recovery programme.

Battershill said the researchers were fortunate to have pre-disaster information and in being able to quickly capture more where there were gaps. Matauranga Maori, traditional Maori knowledge, had also been valuable.

A long-term biodiversity study at the entrance to the Port of Tauranga had been hit by oil, enabling results to show that in just over six months there was "quite a significant clearance of oil patches" and now the fauna had returned to "pretty much what they were like before".

All the Bay of Plenty beaches had been surveyed with no "massive kill" of shellfish found, even in the most highly oiled areas.

In a relatively short period there was a return to background levels of kaimoana species and detailed examination of rocky reef systems had found "basically no disturbance".

Waikato University associate professor Nick Ling said strong public concern had led to lab studies on the effects of oil and dispersant on snapper and red rock lobster.

These showed that "the acute effects of exposure to heavy fuel oil with or without Corexit are relatively small and transitory".

"We got off very lightly with this shipwreck," Ling said.

"Not only was there a huge volunteer army that cleaned up a lot of the oil, because it ended up on sandy beaches it was easy to clean up as opposed to rocky reefs where it is very difficult.

"We got off very lightly with the cargo as well. There weren't a lot of real nasties on that ship - and about a third of the containers were completely empty."