WELLINGTON - A growing oil spill from a stricken container ship sent new clumps of fuel oil on to one of New Zealand's most popular beaches on Tuesday in the country's worst environmental disaster in decades.The 47,230-tonne Liberian-flagged Rena has been stranded on a reef 12 nautical miles off Tauranga on the east coast of New Zealand's North Island since running aground last week. Heavy seas were preventing attempts to steady the vessel, making it dangerous to remain aboard.RELATED:Nahal Zin operations halted following huge oil spillAuthorities said up to 300 tonnes of the ship's 1,700 tonnes of heavy fuel oil had already escaped. Authorities have posted bright yellow notices on beaches telling bathers to stay away and not to eat seafood."The situation over the coming days, from an environmental perspective, is going to get significantly worse," Environment Minister Nick Smith told a media briefing."The government is determined ... to throw everything possible at minimising the environmental harm of what is now clear to be New Zealand's worst environmental disaster in many decades."The district's long, golden beaches are a magnet for surfers and nearby waters have an international reputation for big-game fishing. "People are angry that this could have happened on our doorstep and it could really ruin one of the best beaches around," resident Jim Kohu told Reuters.Clean-up teams positioned containers and large plastic bags to scoop up the large black clumps washing ashore, as volunteers joined in to help with supermarket bags and shovels.Booms have been placed over some harbour entrances to keep oil out of wetland and wildlife habitats. Around 50 seabirds have died and teams of naturalists have scrubbed and treated 20 more for oil contamination."The next 24 to 48 hours are pivotal if an environmental catastrophe is to be averted," said World Wildlife Fund marine manager Rebecca Bird.Maritime New Zealand said weather overnight had shifted the ship on the reef and continuing heavy swells and strengthening winds were making it too dangerous to stay on board."All personnel have now been taken off the vessel as a precautionary measure due to the conditions," the agency said in a statement.Swells of up to five metres (15 feet) and winds gusting up to 25 knots were battering the ship, shifting around on the reef. The ship, holed and flooded in its two forward compartments, was listing about 15 degrees, but authorities said there seemed no danger of it breaking up."There has been no change to the structural integrity of the vessel, which is described as being in relatively good shape," Maritime NZ said. Pumping fuel off the ship on to a barge, it said, would resume as soon as conditions allowed. Refloating and salvage of the ship are the responsibility of the owner, Daina Shipping, a unit of Greece's Costamare Inc. , and salvage experts, but any plan needs official approval.The ship was en route to Tauranga, 200 km (120 miles) southeast of New Zealand's biggest city, Auckland, the country's biggest export port and a hub for transshipping cargo, to collect cargo before heading for Singapore.