Energy Minister Judith Collins has released the terms of reference for the inquiry she recently announced to determine whether New Zealanders are paying a fair price at the pumps.
She made the call after it was revealed that company margins - the difference between what they pay for petrol and what they sell it for - had widened over recent years.
"We need to know why this is happening and determine if what people are paying at the pump is reasonable, or whether the companies are making super-profits," she said on Friday.
The inquiry would take "an in-depth look at fuel company finances" to get answers.
Ms Collins described the inquiry as "technical" but said she expected "some observations" to be made about regional price differences.
The inquiry will be conducted by the Ministry of Business, Innovation and Employment, with consultants helping.
Ms Collins said the four big companies - Z Energy, BP, Mobil and Gulf - had all assured her they will cooperate fully.