BP New Zealand is selling down part of its holding in the nation's only oil refinery operator, NZ Refining, potentially reaping as much as $86 million.

BP New Zealand Holdings and Europa Oil NZ told NZ Refining after the market closed on Thursday that it planned to sell as much as 11.1 per cent of the company, reducing its holding to 10.09 per cent, NZ Refining said in a statement to the stock exchange this morning.

NZ Refining's stock is in a trading halt pending confirmation from BP that it has sold the shares, the statement said.

NZ Refining shares last traded at $2.49, valuing the 11.1 per cent stake at $86.4 million.

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BP's reduction in shareholding won't affect existing contractual arrangements between NZ Refining and BP, NZ Refining said.

Among NZ Refining's other major shareholders, Mobil Oil New Zealand owns 17.2 per cent, and Z Energy owns 15.36 per cent, according to NZX data.