Labour's energy spokesman is questioning whether we are being taken for a ride, when it comes to fuel price margins.

Figures released from the Ministry for Business, Innovation and Employment show petrol and diesel margins are at their highest levels in almost 20 years.

It comes amid concerns of Z Energy monopolising the market, after the company bought Caltex New Zealand in April for a majority stake in the retail fuel industry.

That gave them more than 50 percent of the retail fuel market.

Stuart Nash said a lack of competition could be to blame, and wants new Energy Minister Judith Collins to investigate.

"If the margins were really skinny then you could understand the price, but the margins are higher than they've been for a long, long time. You sort of get the feeling that were being ripped off to a certain extent I think."

Nash said Ms Collins, who was given the portfolio in Bill English's Cabinet shake-up, should rise to the challenge.

"I would think this is the perfect opportunity for Judith to say 'hey you know what, let me stamp my mark in this area, let me do something that kiwis will appreciate, lets investigate what is going on here'."