Z purchases its crude from Shell and coordinates imports with the refinery and rival Mobil Oil New Zealand to optimise the operation at Marsden Point. It also has a 15.4 per cent stake in the plant.
Buying spot fuel cargoes compounded the impact of rising crude oil prices on the firm's costs and retail fuel volumes. A weaker New Zealand dollar also contributed.
Brent crude briefly pushed through US$80 a barrel in mid-May and Z's main port price for 91 octane petrol reached a record $2.29 a litre that month as a result. October crude was trading at US$71.70 earlier today.
Z's commercial diesel, jet fuel and bitumen sales for any month are priced using inputs in the prior month. A US$1 movement in crude prices generally moves earnings by about $1m.
The company noted that Brent crude increased about 14 per cent during the June quarter to end the period at about US$78/barrel. During the same period the New Zealand dollar fell almost 7 per cent to 67.51 US cents.
Not only were margins compressed, but industry volumes for the quarter also fell about 8 per cent from in the March quarter as homes and businesses reduced purchases.
Z says those impacts, which reduced operating earnings by about $10m, are temporary and the company expects to be able to moderate some of the volume loss during the rest of the year.
Lower crude oil prices may also enable the company to recover some of the adverse effects from the June quarter.
The shares last traded at $7.50 and have declined 2.2 per cent so far this year.