Greenpeace has called on authorities to answer "serious questions" over the chemical used to disperse oil from the Rena, after a top environmental adviser yesterday admitted scientists were "in the dark" about its effects.
Maritime New Zealand sprayed about 200 litres of Corexit 9500, the chemical used during the large spill in the Gulf of Mexico last year, to try to break down oil from the Rena in the early days of the disaster.
Authorities eventually scrapped the operation after it proved ineffective against oil floating on water.
Its use sparked outrage among environmental groups, some scientists and veterans of the gulf spill.
One United States resident describing it as a "toxic chemical soup" that fouled seabed habitats in shallow water.
Many Tauranga residents have also voiced concern and at one public meeting last month, some shouted "it's banned overseas" as Environment Minister Nick Smith defended its use.
The holder of Waikato University's chair in coastal science, Professor Chris Battershill, said the coming months would reveal the environmental impact of the dispersants.
"This is the first big oil spill in New Zealand's history. We've quickly realised what isn't known in this country about oil dispersants and toxicology.
"Unfortunately we know little about the toxicology of the dispersants on New Zealand species. Right now we need to learn more about the coastline and the food chain implications."
More than 20,000 birds are thought to have fallen victim to the Rena's oil, and a ban on the collection of seafood is still in place.
"The relevance of dispersant toxicology on New Zealand species is a huge gap in our knowledge," Professor Battershill said.
"We are using similar dispersants used in the Gulf of Mexico disaster and are in the dark as to the short-term lethal effects versus the long-term effects on the food chain and ecology."
Greenpeace spokesman Steve Abel said the group was "very concerned" when it learned Corexit 9500 was being used.
"There certainly needs to be some serious questions asked as to how we would react to other spills, given the dispersants used by Maritime New Zealand weren't having any effect and only added to the toxic load borne by the marine life.
"I hope that out of any research the university gives to the authorities will come advice against the use of Corexit."
Green MP Gareth Hughes, whose party has called for a royal commission of inquiry into the disaster, described Professor Battershill's comments as "alarming" and said dispersant should not have been used at all.
"Given the lack of information on its impact, the Government should have adopted the precautionary principle not to use it," he said.
"Its use was a trial and a risk the Government took in the early days ... the fact is New Zealand should not be a trial place. The great fear is that Corexit could be making the problem worse in the long term."
Professor Battershill said the university would soon have a clearer picture of how long the marine environment would take to recover.
CONTAINER UNLOADING OPERATION STALLS AS STORMS LINGER
The operation to unload the Rena's containers remained on hold last night, as strong winds lingering from a bout of bad weather yesterday continued to whip up seas around the stricken cargo ship.
Authorities said the ship had survived the latest storm without further damage, but its condition remained "fragile" and it was at the mercy of the sea.
Salvors were monitoring the ship last night from the tug Go Canopus, which remained connected to it.
The most precariously positioned containers, which had been leaning in a pancaked stack at the Rena's stern, were among nearly 100 removed by container recovery company Braemar Howells.
The company also agreed to reopen a section of beach at Papamoa that had been fenced off for "last resort" use as a temporary depot to bring badly damaged containers ashore.
But a 50sq m area of beach would remain fenced off to store oil spill response equipment, machinery and decontamination materials.
Experts were meanwhile preparing to release more wildlife as their habitats were cleaned.
A team is due to visit Rabbit Island tonight, where the first little blue penguins were released on Tuesday.