New Zealand Refining is investigating whether it can bring forward a project to improve the shipping channel into Whangārei harbour by a year.

The widening and straightening of the channel will allow users of the Marsden Point refinery to deliver crude oil in fewer, but larger tankers, and could lift the refinery's margins on each barrel by US30 cents.

Chief executive Mike Fuge says the project is "very exciting" and the company is reviewing its tank maintenance schedule to see if it can have the extra storage needed to cope with the larger cargoes available earlier.

If that can be achieved, the dredging could possibly be carried out during the 2020-21 summer.

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"The only question is, can we bring it forward? There's an opportunity there," he said. "I'm not going to promise that we can."

Marsden Point is the country's only oil refinery and produces about 70 per cent of the petrol, diesel and jet fuel used in New Zealand. It is 43 per cent-owned by Z Energy, BP and Mobil.

The total cost of the dredging project is estimated at $60-70 million, with the dredging component of that expected to be about $35m. A year of environmental monitoring is required before the project can get underway, and that work will start in the next couple of months.

Fuge said the storage challenge reflects how "incredibly optimised" the existing refining operation is, with about 60 per cent of every cargo going into the processing stream.

The company, which trades as Refining NZ, earlier reported a 62 per cent slide in full-year profit after a major shutdown reduced throughput and regional processing margins fell.

Net profit fell to $29.6m in the 2018 calendar year, from $78.5m in 2017. Revenue fell 13 per cent to $359.6m, with average margins declining to US$6.31 a barrel from US$8.02 the year before. Throughput fell to 40.44 million barrels, 3 per cent less than a year earlier.

As signalled, the site's largest shutdown since 2004 reduced full-year earnings by about $43m after running over schedule. Lower margins trimmed earnings by about $12m while higher power costs and remediation work on the firm's fuel pipeline to Auckland increased costs by about $3m.

A lower New Zealand dollar, and a record 21 million barrels shipped on the pipeline to Auckland, recouped about $9m of that. The company's shares fell 4.4 per cent to $2.16, taking their loss for the past year to 10 per cent. Investors will receive a 4.5 cent final dividend on March 21, down from 12 cents a year earlier.

Fuge said 2018 had been a tough year but the record throughput and strong operational performance in the second half "is very encouraging for the years ahead".

With no major shutdowns planned in 2019, he said the company "fully expects" to lift its operational performance further and achieve throughput of a record 44 million barrels this year. The current record was 42.67 million barrels in 2016.

The firm has maintained a raft of small upgrade projects since completing the installation of the $365m continuous catalytic regeneration unit in late 2015. It continues to invest to ensure it remains competitive against larger, more modern refineries its customers can also buy product from.

Fuge said the company is re-thinking the final element of a three-stage project to increase the capacity of its fuel pipeline to Auckland. A drag reducing agent it is planning to trial in the third quarter may increase capacity by 15 per cent at lower cost.

The company is also considering a $10m investment to increase bitumen production.