A cargo ship which ran aground off the coast of New Zealand three months ago has broken in two, spilling containers and other debris into the sea.  Officials said heavy seas had snapped off the stern section of the Greek-owned Rena, which leaked large amounts of oil on becoming stranded in October, according to the (BBC).  Clean-up teams have been alerted amid fears the ship could leak some of the remaining oil on board.  The accident has been described as New Zealand\'s worst maritime disaster.  The Rena struck the well-marked Astrolabe Reef off the North Island resort area of Tauranga on 5 October.  \"Overnight, the vessel was hit by wave action of up to 6m, heavy winds and heavy seas. That\'s caused the ship to break into two parts,\" Maritime New Zealand spokesman Ross Henderson said.  Salvage crews have removed more than 1,100 tons of oil from the stricken vessel, but 385 tonnes remain onboard.  Hundreds of tonnes of fuel have already spilled into the sea, killing more than 20,000 seabirds in the area.  The captain and other senior officers face up to 16 charges relating to the wreck.