There's a battle brewing between the big oil companies and the taxman.

Customs is demanding excise duties going back 30 years on leftover fuel in a pipeline between the Whangarei refinery and Wiri in South Auckland.

According to Customs, more than $100 million is due in unpaid duties and penalties.

The pipeline is jointly owned and operated by Z Energy, BP, Mobil and Chevron.

Z spokeswoman Sheena Thomas said they have been blindsided by the tax bill.

She said all of a sudden they received a letter saying that they owe not just unpaid duties but penalties dating back to 2007.

"We don't think it is fair and reasonable and it has come out of the blue a little bit because we have been really cooperative."

Z is planning to challenge the retrospective tax demand.