Forecast increases in demand from Auckland transport users prompted Refining NZ to invest millions increasing the flow-rate of its fuel pipeline.

Refining NZ increased the pressure of its Auckland fuel pipeline weeks before it burst, without carrying out a fresh internal check on the pipeline's integrity, the company has confirmed.

The NZX-listed firm is spending more than $5 million this year upgrading the pipeline and increased the volume of fuel that could be pumped through it by 5 per cent in early August, shortly before it ruptured on September 14.

However, the company said there was no evidence the pressure increase to 87 bar was the straw that broke the camel's back.

REFINING NZ Refining NZ believes its Auckland fuel pipeline was damaged a few years ago, but it did not rupture until last month.

It says the pipeline rupture – which to led to jet-fuel rationing and dozens of flight cancellations at Auckland airport – was due to damage by a digger which it believed may have deeply scraped the pipe as early as 2014.

Refining NZ has commissioned "asset integrity expert" Quest Integrity to carry out an investigation.

"We're pretty confident that their findings will concur with our initial conclusions that, given the extent of mechanical damage, the pipeline would have failed regardless of the operating pressure," spokesman Greg McNeill said.

"There is no evidence of a causal link between the August upgrade and the pipeline failure."

READ MORE: Damaged Auckland fuel pipeline may have been waiting to break since 2014

The pipeline was "complex" and an electricity glitch or an issue with a valve or pump along the pipeline could instead have been the final trigger, he indicated.

The company last ran a scanner through the pipe in 2014 and McNeill defended its decision not to do so again before turning up the pressure, even though it has confirmed a scan would have detected the digger damage.

Refining NZ was only required to test the condition of the pipe's walls by running a scanner through the pipe every 10 years, he said.

Lloyds Register recertified the underground pipeline was able to carry the extra fuel, based on data from the 2014 scan and visual checks, he said.

Chief executive Sjoerd Post indicated last week that Refining NZ might nevertheless review its testing policies in the wake of the leak, which it has forecast will cost the company between $10 million and $15m. "We would have to consider inspection frequency and the like," he said.

The company has also decided to reduce the pipeline's pressure to 70 bar until the end of the year as a precaution.

Refining NZ is spending millions of dollars to pump fuel through the pipeline faster so it can meet the growth in demand for fuel from Auckland transport users.

The company's original plan had been to increase the flow-rate by more than 60 per cent to 415 cubic metres per hour through a five-stage upgrade that involved improved pumps and adding a "drag reducing agent" to petrol and diesel to reduce friction in the pipe.

But Post said it had decided not to use the fuel additive because that had been unacceptable to customers.

The latter phases of the original upgrade programme were superseded by a "new three-phase capacity upgrade programme", the first phase of which was the one completed in August, McNeill said.

The second stage of that project is due to be completed by the end of the year, with the final phase scheduled for 2019. Refining NZ earmarked $5.1m for phases one and two.

Refining NZ still did not know who damaged the pipeline, McNeill said. Post believed swamp Kauri had been removed from the land around the pipeline at some point but said it had no proof of that.

The statutory inquiry into the fuel leak was the responsibility of Northland Regional Council, he said.