More oil from a cargo ship that has been grounded on a New Zealand reef for two months is expected to wash up on the country\'s beaches after the wreck was battered by recent strong swells, shipping authority officials said Sunday. The oil had slowly leaked from the duct keel, a system of pipes running along the bottom of the ship, since the Liberian-flagged Rena grounded and had probably been exposed to sea water for some time, a Maritime New Zealand (MNZ) official told the Fairfax news organization. MNZ national on scene commander Mick Courtnell said salvors working on the Rena Saturday saw blobs of oil floating from the wreck, which has been stuck on the Astrolabe Reef, 12 nautical miles off the east of the North Island, since Oct. 5. \"We followed this up with two observation flights and our team estimates a small amount - perhaps half a tonne - of weathered oil has been released, probably from the duct keel,\" he said. Courtnell said oil spill response teams would be on standby on beaches in the Bay of Plenty region to clean it up over the coming days. Strong winds were forecast again Sunday and the swell was predicted to reach up to three meters early Monday, but weather was set to ease in the area mid-week. \"With the continuing swells we may see more of this oil come out of Rena,\" Courtnell told Fairfax. Last month, animal rescue workers began releasing seabirds back to the wild after they had been cleaned and treated for oil contamination, and beaches in the Bay of Plenty region were reopened to the public. Two senior officers from the Rena\'s Filipino crew have been charged in connection with the grounding and the resulting oil contamination.