Brent crude oil prices dropped more than 40 percent late last year, from about US$86 ($127.33) a barrel in early October to US$51 in late December. March Brent was recently at US$61.36.
Wellington-based Z Energy said its retail and commercial fuel trading improves in a falling crude market.
Lower prices encourage retail customers to buy more fuel, with average fill volumes recently rising to the more typical 28 litres, from a low of 26 litres in the September quarter.
A one-month pricing lag on some commercial fuel sales also advantages the firm when product prices are falling.
The $10m negative impact of rising crude prices on fuel margins that Z disclosed in its first-half earnings "has more than fully reversed" to be a positive $8m "price lead" in the December accounts, the company said in a statement on NZX.
Z Energy said total industry fuel volumes of 2.51 billion litres in the December quarter were 5.2 per cent higher than a year earlier. That growth was primarily driven by commercial markets.
Z group sales, excluding exports and other sales to industry, increased 0.7 per cent.
Industry retail segment volumes fell 3.1 per cent in the quarter, but Z Energy said its volumes were unchanged.
The company said its commercial volumes grew in the quarter, but it lost market share in diesel and jet fuel sales.
"The decline in commercial diesel was from high-volume, low-margin account losses which have been replaced towards the end of the quarter by volume that has improved the portfolio mix."