The Automobile Association says diesel prices should be going down, not rising along with petrol.
AA Petrolwatch says petrol prices rose by 10 cents in April, and 91 octane fuel was now $2.03 at most outlets.
Diesel rose 9 cents to $1.29 a litre.
It said the increase was being driven by rising oil prices, but fuel companies had been too quick to put up prices at the pump, particularly for diesel.
Petrolwatch spokesperson Mark Stockdale said the margins fuel companies were making on diesel were well above average, and in fact they should be dropping its price.