Z Energy, the country's biggest fuel retailer, has cut full-year earnings guidance by $30 million due to an extended shutdown at the Marsden Point oil refinery and high crude prices in the June quarter.

The company is now forecasting operating earnings of $420m to $455m for the year through March 2019, down from previous guidance of $450m to $485m in earnings before interest, tax, depreciation and changes in financial instruments.

The Wellington-based company attributed about $20m of the earnings reduction to a longer-than-expected shutdown at New Zealand Refining's site at Marsden Point. The refinery's hydrocracker had been scheduled for a 39-day shut ending on June 11, and that was initially extended to June 20 but further delays prevented the refinery resuming full output until July 6.

"The delay to restarting the refinery has meant a reduction in refining margin for the quarter, losses from unplanned fuel oil exports, and necessitated increased volumes of imported product purchased on the spot market rather that Z's advantaged term contract pricing," Z said in a statement filed with the stock exchange.

Z, which acquired the rival Caltex business in 2016, reported a 6 per cent increase in replacement cost ebitdaf to $449m for the year ended March 31. Net profit rose 16 per cent to $205m on the same basis as the firm benefited from strong jet fuel and diesel demand and ongoing synergy gains from its acquisition.

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.