TAURANGA, New Zealand, Oct 11, 2011 (AFP) - New Zealand said Tuesday an oil spill from a container ship stranded off the North Island coast has become the country's worst maritime pollution disaster. The amount of oil spewing from the stricken vessel Rena, which hit a reef last Wednesday, had increased five-fold after it sustained further damage in a storm overnight, Environment Minister Nick Smith said. "I'd like to acknowledge this event has come to a stage where it is New Zealand's most significant maritime environmental disaster," Smith told reporters at Tauranga, where once-pristine beaches have been fouled with oil. He described as tragic the latest developments, in which up to 350 tonnes of heavy fuel oil leaked into the Bay of Plenty early Tuesday, but said there was little authorities could do to prevent it. "It is my view that the tragic events we are seeing unfolding were absolutely inveitable from the point that the Rena ran onto the reef in the early hours of Wednesday morning," he said. The latest spill dwarfed an initial leak of 20 tonnes after the Liberian-flagged vessel ploughed into the reef, 22 kilometres (13.6 miles) offshore. The crippled ship ruptured a fuel tank after it was pounded by five-metre (16.5-foot) swells, forcing a salvage crew on the vessel to issue a mayday earlier Tuesday and evacuate as a safety precaution.