The Labour Party is calling for a parliamentary select committee inquiry into the prices petrol companies charge.
Labour said New Zealanders were paying far more for their fuel than motorists in most other countries.
Energy Minister Simon Bridges recently wrote to petrol companies expressing his disappointment at their profit margins in late 2014.
The Government now monitors margins weekly because, as Mr Bridges puts it, "sunlight is the best form of disinfectant".
But Labour's energy spokesperson Stuart Nash said writing warning letters was no way to regulate an industry.
"If the Minister thinks writing a letter to oil companies is a way to regulate an industry or bring them into line then he's completely wrong, it's like a slap across the wrist with a wet bus ticket."
Mr Nash said a claim by Z Energy that the Government was using inaccurate data to calculate petrol companies profit margins should also be investigated.