The New Zealanders that were picked up at sea by the Achilles are still in custody, but it is not supposed that the authorities will be able to do anything with them. At present the only evidence against them, in addition to the fact of their having been picked up at sea in the Ploughboy's boat, is a letter, which we received from our Correspondent at New Zealand, stating that some New Zealanders on board the Ploughboy had murdered five of the crew and then escaped in one of their boats.