A World War II minefield believed to have been laid off the entrance to Lyttelton Harbour on Banks Peninsula, has eluded navy searchers as port authorities plan to dredge a deeper channel.
The 10 mines, thought to weigh up to 1000kg each, were apparently laid by the German minelayer, Adjutant, in June, 1941.
They were not moored but lay on the seabed and were detonated by the acoustic or magnetic activity of ships passing overhead.
None of the mines exploded and no ships were sunk and the navy believes over the years the mines sank into the "glutinous ooze" of the seabed.
"It is probable they were either fused incorrectly or had a defective initiation mechanism," said the head of the navy's Mine Counter Measures Team (MCMT), Lieutenant Commander Kelvin Barrett.
The navy was asked to search for the mines amid fears there may be a problem dredging the channel if the mines were still there. Lyttelton Port of Christchurch planned to deepen the channel by up to four metres.
The MCMT worked from the navy dive ship HMNZS Manawanui earlier this year and searched about a square kilometre of seabed, although Lt Cdr Barrett said they did not have the capacity to find mines below the seabed. If they were not visually sighted they would not be found.
Lt Cdr Barrett told NZPA while there was not a great chance of the mines exploding if they were picked up in the dredge, when they dried out they could become dangerous.
"It is likely they may well have corroded.
"They are probably inert in that they are wet and the fuses and the like are unlikely to function while they remain wet."
He said if a dredge picked up a mine and dumped it in its hopper it was "probable it won't initiate (blow up).
"But if it was allowed to dry out and that would take several weeks, certainly the detonators become quite dangerous."
He said if a mine was picked up in the dredgings the military would be called to safely dispose of it.
Adjutant also laid about 10 similar mines off the entrance to Wellington Harbour a day or two after laying the mines at Lyttelton and that minefield had also not be found, said Lt Cdr Barrett.
Adjutant later met the German auxiliary cruiser Komet north of the Chatham Islands but broke down and, after being stripped of anything useful, it was scuttled.
The navy said Adjutant had the distinction of being the only enemy vessel sunk in New Zealand waters during the war.