The AA is questioning the latest rise in diesel prices.
The price of both diesel and petrol rose by three cents a litre yesterday. That followed a three cent increase on Friday.
The national price for diesel is just under $1.12. Unleaded 91 is retailing for just under $1.87.
However it is possible to find petrol being discounted by more than ten cents a litre in some areas, like south Auckland.
The AA says refined petrol commodity prices have risen by six cents per litre since last Tuesday, which matches the petrol price increase at the pumps.
But it says diesel commodity prices have only increased by 2 cents per litre. It does not believe the price rise was justified.
NPD petrol and diesel pump in Christchurch (Newshub.)
Diesel in Auckland at $1.059 (Newshub.)
Breaking down the cost of fuel
The petrol companies say a range of factors determine the pump prices.
Z says it buys crude oil, as well as refined diesel and petrol. The amount it pays covers the cost of petrol and diesel on international markets and the cost of shipping the fuel to New Zealand.
There are also government taxes and levies. At the moment 67.13 cents per litre of petrol is collected in fixed excise tax. In addition, GST of 15 percent is collected on the overall price of fuel.
The companies say they also have operating costs (staff wages, electricity, etc) and what is left over is their profit.
Z says that after it pays for fuel, taxes, levies and operating costs it makes a profit of between 4 to 6 cents per litre.
Z petrol station in Christchurch (Newshub.)