A container ship stranded on a reef off New Zealand\'s North Island is leaking oil and has created a two kilometre (1.2 mile) slick, maritime authorities said Thursday. The 47,000 tonne container vessel \"Rena\" hit Astrolabe Reef, about 12 nautical miles (22 kilometres) off the coast of Tauranga, early Wednesday, Maritime New Zealand (MNZ) said. The agency said the Liberian-flagged vessel shifted in heavy swells on Wednesday night and began leaking oil into the sea. \"An aerial observation flight this morning identified the slick, which stretches from 1,500 to 2,000 metres from the vessel in a narrow ribbon.\" it said. It was not clear how much of the 1,700 tonnes of heavy fuel oil on the vessel had leaked. MNZ on-site controller Rob Service said a wildlife aid centre for birds and animals potentially affected by the spill was being set up and experts were assessing whether dispersants could be used to break up the slick. \"We are ready to launch whatever level of response the situation requires,\" he said. The Astrolabe Reef is covered with colourful sponges and anemones, according to the Department of Conservation, with seals and gamefish such as marlin common in surrounding waters. None of the ship\'s 25-man crew were injured when it ran aground. Why it stranded remains unknown and is subject to a Transport Accident Investigation Commission inquiry.