Increasing profit margins for petrol companies has led to calls for an inquiry.

Import margins have doubled in the past four years, from 20 cents to 36 cents per litre for petrol, and from 22 cents to 40 cents for diesel.

Labour's energy spokesperson Stuart Nash said it looks like price gouging, and if there's a reason, the companies need to provide proof of it.

"When prices drop, they're very very slow to drop their prices. When prices increase they are incredibly fast to put them back up.. and we don't question it."