With the rip mended in the Ruakaka to Auckland fuel pipeline the focus now is on how it was caused and by whom.

The Northland Regional Council is investigating how the damage was caused in light of rules protecting the 2m-deep, 170km-long pipeline.

"We have turned out attention from the recovery and clean-up to try to find the cause of the pipeline rupture," said NRC group manager regulatory services Colin Dall.

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"It's got to be done properly."

The council had earlier concentrated on the clean-up process and monitoring any effects of the spill on the environment.

The Government, which had talked up the likelihood of major disruption to air travel and Auckland traffic and called the pipeline failure "an embarrassment", has not decided yet whether to commission its own investigation into the pipeline outage and its effects.

"Any decision around an investigation, or what form it would take, would be directed by the next Government," said a Ministry of Business, Innovation and Employment spokesman.

Refining New Zealand (RNZ) owns and operates the pipeline, while the refining company itself is owned largely by fuel companies Z Energy, Mobil and BP.

RNZ chief executive Sjoerd Post said there would be a review of systems and the repair process to learn if Refining NZ could do better should the need arise again.

''We believe that we have complied with our legal obligations in responding to the incident. Our focus now is upon getting back to our day-to-day business.

''We are investigating the circumstances in which the damage to the pipeline occurred and whether there is any more that can be done by ourselves and others to prevent a similar incident in future.''

The company's internal inquiries on the incident began almost straight away and are ongoing.

But it said it had nothing to say about any external inquiry: ''That would be for other parties to comment on.''

Photos of the exposed broken pipe at the centre of last month's 10 day clean-up and repair job showed what appeared to be scrapes, possibly caused by a digger blade.

The damaged pipe is being examined by metallurgical experts to establish the cause of the initial rupture and the corrosion process that followed.

Residents overlooking the land where the 70,000 litre spillage occurred said they had not seen a digger working there in recent years. Kauri excavation was not uncommon in other parts of the former peat swamp that is now farm pasture.

The pipeline is marked by white posts in paddocks, hazard signs on roadsides and fences and its location is signalled on every affected property's local council Land Information Memorandum (LIM) and other documents.

The Whangarei District Plan rules prohibit fences or structures above the 275mm pipe, planting trees over it, digging deeper than 40cm or doing anything on the designated corridor that could affect the pipeline.

Written consent by Refining NZ is required for any trenching and excavation or drilling in the corridor.