An initial investigation by the Marsden Point oil refinery has found a digger driver searching for swamp kauri caused the critical damage to the fuel pipeline months ago.

When the pipeline burst on Thursday, the response has included finding out how the damage had been caused.

A source familiar with the situation said refinery bosses had found a swamp kauri hunter had been working in the same area as the damaged pipe around three months earlier.

In his efforts to remove a log 20 metres long and one metre wide, the digger struck the fuel line.

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It did not rupture but laid the seed for the failure that would follow.

While the route of the pipeline is clearly marked with warning signs, the source said that the signs closest to where the pipeline ruptured where overgrown and could not be clearly seen.

The digger driver moved on and months passed until Thursday, when refinery bosses increased the pressure in the pipeline which forces fuel to Auckland.

The increase in pressure was too much for the damaged pipeline and it ruptured, spewing fuel into a peat swamp.

Refining NZ spokesman Greg McNeill said that the pipeline burst on Thursday as a result of "external damage" in a peat swamp.

He could not confirm the cause of when the damage happened but said excavation teams had found kauri near the pipe.

"There was swamp kauri on the site, in our excavation we have come across pieces of it," he said.

"The team can see that there is quite a series of scrapes across the top of the pipe. What actually led to that I don't know."

Swamp kauri extraction is controlled by local councils.

Whangarei Mayor Sheryl Mai said she had not been briefed on the Ruakaka work but would be asking questions.

"It's a very serious event that has happened in our district. The Refining Company is the lifeline for all of our fuel and clearly this is a major disruption, so I will be asking some questions of our staff to clarify what should have happened and what went wrong."

The pipeline damage in a peat swamp at Ruakaka. Photo supplied
The pipeline damage in a peat swamp at Ruakaka. Photo supplied

A review of NZ oil security for the Ministry of Business, Innovation and Employment recognised in 2012 that any cut in the oil pipeline would mean "no jet supply to Auckland Airport".

"In the short term there are few alternatives for jet supply into Auckland," the report said citing options like diverting flights to other airports - here or overseas - for refuelling before landing in Auckland.

The report considered options including duplicating the pipeline's Auckland terminal at Wiri, but said that would cost more than $10 million a year and the probability of a short-term disruption to the pipeline was put at only 0.5 to 1 per cent a year, or once every 100 to 200 years.