Z Energy, the service station chain floated by Infratil and the New Zealand Super Fund two years ago, warned it may be facing millions of dollars of additional costs as the nation's customs service seeks to claw back excise duties stretching over the past three decades on leftover fuel in a pipeline from the Whangarei refinery. New Zealand Customs Service is looking into what it considers to be unpaid duties on fuel transported from the Whangarei refinery via a pipeline to the Wiri Oil Services terminal in South Auckland, which is jointly owned and operated by Z Energy, BP, Mobil and Chevron.