(CNN) -- The stern of a cargo ship that ran aground last year on a reef off the coast of New Zealand has slipped further underwater, officials said Tuesday, spewing nearly 10 more tons of oil into the sea.
Some 21 containers holding a hazardous metal called cryolite -- a by-product of aluminum smelting -- have also slipped off the vessel as it plunged downward, said Maritime New Zealand spokeswoman Michele Poole.
"We had significant storms last weekend and the seas really have not subsided," added Poole. "So it's very rough out there."
Authorities say they do not consider the metals dangerous because they are wrapped in plastic within the container. Should the containers break apart, Poole noted, the metals' toxicity levels would likely be diluted by the surrounding sea water.
The metals are not thought to pose an environmental risk, she said.
Still, emergency management teams were again placed on alert after monitoring devices aboard the ship -- named the MV Rena -- indicated Tuesday morning that the vessel had further slipped off the Astrolabe Reef, located about 12 nautical miles off the city of Tauranga.
Authorities said six little blue, or korora, penguins have since been recovered after new pockets of oil surfaced later Tuesday, but added that they do not expect a need for a major recovery effort.
The Liberian-flagged vessel struck the reef in October and has split in two.
About 300 tonnes of oil have since leaked from the ship's hull, prompting the country's environment minister to call the spill New Zealand's most significant maritime environmental disaster.
Between 200 and 300 containers, comprised mostly of wood and milk products, were also lost overboard when the ship broke apart
"There is inevitably going to be a lot of mess and disturbance for the next few days while this is cleaned up and we would like people to use common sense and stay well away from the debris -- in some cases this will mean staying out of the water as well as away from material washed up on the beach," National On Scene Commander Alex van Wijngaarden said in a statement.
Debris from the vessel washed up on a local beach Monday, and a sheen of oil remains visible around the vessel. But officials say they remain wary of committing personnel, particularly underwater crews, to the crash site.
"The wreck is highly unstable," noted Poole. "So it's not worth us risking sending divers down there."
A barge equipped with special cranes, shipped from Singapore, has since taken part in recovery effort, she added.