The discovery of a jet fuel leak on a Northland section of the pipeline from the Marsden Pt oil refinery to Auckland has forced the evacuation of a couple from their home.

Refining NZ spokesman Greg McNeill said a pressure drop on the refinery to Auckland multi-product pipeline, that runs 170km to Wiri, was detected at 2.30pm on Thursday.

The leak was discovered about 8km south of the refinery at Marsden Point near Ruakaka.

Immediately the section of pipeline was isolated and valves on either side of the leak were shut off.

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"We've isolated and contained the leak and our crew is busy carrying out a recovery and repair operation," Mr McNeill said.

The couple - who were evacuated as a safety precaution - were living about 100m from the site and it's likely they would remain at a motel until the repairs were complete.

A team of firefighters normally based at the refinery had gone to the scene and remained on a cordon.

"We're working with the oil companies to ensure that fuel supplies aren't unduly impacted," he said.

Mr McNeill said it was the first leak he had heard of on the section from Marsden Pt to Wellsford and leaks were not common at all.

Yesterday crews were at the scene off Marsden Pt Rd and were hoping to start digging to expose the leak. Until they could pinpoint the leak they were unable to say what had caused it, Mr McNeill said. It was unclear just how much fuel had leaked.

"We won't know exactly what we are dealing with until we expose the pipe and see the damage."

The refinery was continuing to operate while repairs were being carried out and it was not expected the incident would have a significant financial impact on the company.

Refining NZ had reported the spill to the Northland Regional Council.

NRC compliance and monitoring manager Tess Dacre said the fuel had leaked into a farm drain and a "coffer dam" had been put in place to stop the fuel flowing out to the sea.

More absorption materials had been placed down from the dam to ensure all the fuel was captured.

"We are working with Refining NZ to make sure we get the best outcome," Ms Dacre said.