Salvage operations aboard Rena were complicated over the weekend when gases were detected from a dangerous-goods container.
Salvors were able to remove 8.5 containers but one container emitting harmful gas was instead dumped overboard as a precaution.
Maritime New Zealand yesterday reported one container carrying ferrosilicon had been "wet stored" in 9m of water near the Rena after higher than normal gas levels were detected. They said plans were in place for its removal.
Svitzer salvage spokesman Matt Watson said the levels of gas detected were of potential harm to the salvors. The safest option in the circumstances was to submerge the container.
"They've pulled it up and identified some high levels of gas [so] they've dunked it back in the water. It's been a strategic submission as a precaution.
"What they do is they leave it in the water ... and when they are absolutely ready they'll pull it up. My understanding is it's tagged and buoyed," he said.
Neither Maritime New Zealand nor Svitzer were yesterday able to comment on the type of gas seeping from the container, but ferrosilicon is known to produce large amounts of extremely flammable hydrogen gas when exposed to water.
Maritime New Zealand spokeswoman Sophie Hazlehurst said working conditions had been unsafe for salvors with the ferrosilicon container on board. She said an MNZ adviser had approved the "wet storage".
"I know our environmental person said it would be safe stored in the water as it has been," she said.
Mr Watson said he was not yet aware what precautions and process would be used to extract the submerged container when the time came to remove it. Once the container had been brought ashore, question marks remained too around the processing of the container and its potentially-flammable cargo.
Container recovery outfit Braemar Howells' spokeswoman Monique O'Connor said the container would be processed according to its condition.
"There's nothing definitive from Braemar at this stage. How we process depends on the condition of the container when it's delivered into our care," she said, adding it would be premature to comment on specifics.
Containers on board the Rena as well as those in the vicinity of the wreck remain the responsibility of Svitzer. Once they are delivered to shore they enter into the care of Braemar.
Blue twine - enough to fill two skips - was collected from the ocean on Sunday. A helicopter was used to locate the floating twine which was then collected by fast response vessel and barge. The twine was found between Waihi Beach and Mayor Island.
Braemar barges were also used over the weekend to transport not only discharged containers, parts of containers, and twine but also 119 bulk bags of lamb that were moved from the Rena and sent straight to landfills.
Electronic sensor readings on both sections of Rena have indicated no significant changes to the state of the wreck. The sea state is expected to remain around 1m today. A strengthening northerly wind will elevate the sea to around 2m by Wednesday afternoon. Around 2.2m significant wave heights are expected on Thursday.