During the Second World War, there were fears that either Hitler's Germany or the rampant Imperial Japanese Army would invade New Zealand.
Enemy submarines were allegedly seen cruising up Lyttelton Harbour in the early days of war.
Fort Jervois on the harbour's island of Ripapa was garrisoned and the Godley Head battery was set up to counter enemy vessels attempting to bombard the coast.
But ten 1000kg mines were successfully laid undetected by the enemy German minelayer Adjutant in June 1941.
They were not moored but lay on the seabed and were designed to detonate by the acoustic or magnetic activity of ships passing overhead.
Adjutant's minelaying was not successful. Thousands of ships safely passed over the mines during the war - none were sunk - and it wasn't until Germany surrendered in 1945 that the existence of the minefields became known.
The navy believes that over the years the mines sank deep into the seabed.
In 2010, navy dive ship HMNZS Manawanui searched the sea-floor of the harbour entrance after port authorities revealed plans to dredge a deeper channel. The search for the mines came up empty-handed.
A spokesman for Coastguard Canterbury today, however, doubted that the object allegedly seen by the jet-skier was a WWII relic.
"If it was a mine, it could have come from anywhere. It could have come all the way from Germany ... we're talking a long time ago," said the duty officer who was involved in the search.
"Whatever they saw was covered in a lot of marine growth. But who would know what it was.
"They (the jet-skier) suspected it was, the police took it seriously, we had a pretty comprehensive search but couldn't find anything resembling it remotely, so that's it."
The local Coastguard has not previously come across any mines in the harbour.
Police have now contacted Maritime NZ and the incident has been noted as a possible mine sighting.
"It is not possible to confirm what the item may have been," Mr Erasmus said.
