A container load of copper on board the Rena when the container ship struck the Astrolabe Reef in 2011 remains missing.

The 20-foot container of copper held 21 tonnes of scrap, consisting of chopped and stripped household electrical wiring extracted during the Christchurch earthquake recovery. It was carried inside one tonne bulk bags.

The Rena, post-Cyclone Pam. The copper is not expected to be recovered.

Container MSCU6665041 was located at the bottom of Hold 6 and wasn't recovered during cargo recovery operations before the Rena broke in two, and the stern section moved down the reef into deeper water.

Copper is a biocide used in marine anti-fouling paints, but it is also a natural part of the marine environment. Scientists say there's 0.225ppb of dissolved copper in ocean water and about 100ppm in ocean sediments.

There are large natural variations and marine organisms have mechanisms to deal with copper. Copper enters the marine environment both naturally and through human activity, and leaves the ocean in an understood cycle.

A report published this week on the Rena Project page suggests the most likely scenario is that the container was broken open while still stowed at the bottom of the hold after the ship broke in two.

One or more of the one tonne bulk bags was torn open at that time releasing part, or all, of the copper cargo into the ship's holds.

Some copper was found outside the hull on the reef.

The report says: 'The remaining copper is either buried below the debris in Hold 6 or is lying underneath the stern section of the Rena, or both, either within the remains of its container or as a loose discharge.

'In either scenario the majority of the remaining copper is expected to be overburdened by the Hold 6 wreckage, or the ship's aft section, or both, and to be inaccessible.”

The amount of copper released cannot be accurately determined from the amounts located, observed and recovered to date.

Divers found copper over an area of about 20m x 12m outside the ship, at the surface of the sea sediment at 37 metres depth. A smaller amount was found at 28 metres. Both deposits have been removed and amounted to 0.87 tonnes.

Cyclone Pam has since accelerated the wreck's decay. The aft section, including the area that was Hold 6, has collapsed further as the structural hull members degrade and gravity takes over.

This is likely to have had the effect of further encapsulating the remains of the copper container and for it to remain overburdened by the remains of the ship's aft section.

The report adds: 'Within the remains of the ship's structure there is a tangled mess of container and structural debris and the structure itself is now collapsing and compressing encapsulating the remaining debris, as has been shown from the dive surveys.

'Any copper cargo remaining within the ship's structure will not be able to be located.”

Even if the remains of the stern section are removed it is still considered unlikely the remaining copper will be found. The removal process itself is more likely to distribute the copper widely across the sea floor.