About 500 of Rena's containers remain unaccounted for at sea. Some of the errant containers could potentially float away under water, resurfacing a long way from the wreck.
In the aftermath of the storm that broke the Rena, spilling much its cargo in the sea, the locations of around 500 containers remain unknown.
Richard Van Der Spoel, marine specialist in distressed cargo at container recovery team Braemar Howells, said despite the company keeping a near-permanent eye on the sea in the search for strays, many containers remained lost.
"More than 450 containers we've got [identification] numbers for or have reclaimed them, and we know there's about 400 on the bow. So that's 850.
"So [that leaves] around 500 we haven't totally accounted for. We think many of those are around the wreck or in the wreck,'' he said.
Mr Van Der Spoel said that while some containers had been identified and tagged, the majority were unknown and some had the potential to resurface a long way from shore.
"There's none floating at present. Or none that we can find anyway. They've either all sunk or are on the beach.
"[But] it's possible for them to move, depending on the surge and how much they weigh.
"Some of those containers have almost neutral buoyancy. This is the dangerous part of it.''
Having neutral buoyancy means the containers neither sink nor float, but hover between the surface and the sea floor. As water seeps into the containers, mostly those containing canned food, the salt water can react with the products in the confined space and create gases. The lighter gases make the containers positively buoyant, lifting them to the surface.
"There's that possibility, and it might not pop up anywhere close,'' Mr Van Der Spoel said.
Of those containers overboard, Mr Van Der Spoel said there had been a short window to attach tags and buoys before they sunk but most had gone straight under. Braemar has been working to retrieve the containers they have found and are searching every day for more.
To help track the sunken containers Braemar uses sonar scans and aerial flights. They take daily photographs and update maps with the locations of the containers they've found.
"Initially we did twice daily [flights] but at the moment we're not seeing a lot. All the stuff we're seeing is on shore, it's not moving much,'' he said.
The locations of the 32 containers of hazardous goods are also unknown but Mr Van Der Spoel said he believed they would not pose a danger to the public.
He said Braemar believed all 32 containers had sunk and said that those containing ferrosilicon, which can generate hydrogen when in contact with water, remained a relatively low-risk hazard.
While the majority of the containers are expected to have sunk below the Rena, until the sea settles at Astrolabe Braemar's sonar and dive teams are unable to search the area near the ship. Meanwhile, Svitzer divers were finally able to enter the water yesterday to carry out the first underwater survey of the sunken stern.
But Maritime New Zealand national on-scene commander Rob Service said it was still too difficult to complete a full evaluation and, for safety reasons, divers would not be entering the vessel's holds until it was deemed safe to do so. "It's extremely challenging and dangerous work because of the surges of water through the broken structure and the vast amount of torn steel and other debris,'' Mr Service said.
A light oil sheen could be seen around the wreck yesterday. A leak in the boom at Maketu has been repaired, with no oil entering the estuary.
Beach clean-ups continue in Bowentown and Matakana Island at low tide and a Braemar team has also been to White Island.
- Sam Boyer of the Bay of Plenty Times
