Oil from a stranded container ship began washing up on the shore of New Zealand's pristine Bay of Plenty Monday, forcing beach closures at the popular tourist spot, Maritime New Zealand (MNZ) said.

MNZ said "fist-sized patties" of oil from the container ship Rena, which hit a reef last Wednesday, were found on the main beach at Mount Maunganui, one of the largest towns in the North Island's bay area.

It warned people to avoid the toxic discharge, part of a five-kilometre (three-mile) slick which leaked from the Rena accident site 22 kilometres offshore and was blown onto the shore by prevailing winds.

"Public health is erecting signs on the beach warning people to stay away," MNZ said.

The Bay of Plenty's wildlife-rich environment is home to whales, dolphins, seals and penguins.

The Rena remains stuck fast on the reef and, with a storm bearing down, officials fear the vessel could break up and sink, spewing more oil into the sea and creating New Zealand's worst maritime pollution disaster in decades.