Trans-Orient Petroleum says early testing shows potential for up to 12.6 billion barrels of oil on the East Coast.
In comparison, the Tui field is estimated to hold just over 50 million barrels.
The Canadian oil exploration company says the estimate was made after studies of fractured oil-shale sites at Waipawa and Whangai.
In a statement issued on Wednesday, the company said oil and gas seeps from both sites have been assessed and the presence of oil confirmed.
The sites cover nearly 9000 square metres. The company says the tests covered only 10% of the area.
