A rare blue whale is believed to have died at sea before washing up near the northern tip of the South Island.

The massive 27-metre long, 150-tonne beast was found by paua fisherman Philip Walker on Tuesday, near Whanganui Inlet, south of Cape Farewell.

Te Papa marine mammal expert Anton van Helden told the Dominion Post it was rare for the whales to get stranded and it was likely to have died at sea.

There were scars on the whale which indicated it had been "nibbled at" by scavengers, including sharks, he said.

Blue whales are the largest animals known to have lived. From a population of 200,000 in 1900, there are now fewer than 2000.

The Department of Conservation will today photograph the whale, and take tissue samples for Auckland University's whale database.

It will then be left to rot as it washed up in a place too inaccessible to allows its removal.