Satellite imagery has been used to try to find the digger driver who caused the oil pipeline rupture near Ruakaka.
But cloud cover prevented investigators from finding the person responsible for the rupture that caused major jet fuel shortages that saw flights cancelled at Auckland Airport and the navy called in to help move fuel south from Marsden Pt.
Using satellite imagery in the hope of spotting a digger at work was just one step taken to find the culprit.
Read more: Damage to pipeline sparking fuel crisis happened three years ago
Jet fuel flowing through repaired pipeline
With an easing of the environmental monitoring aspect of the September leak, Northland Regional Council (NRC) is concentrating on how and when the break happened.
The council said yesterday evidence it has collected to date indicates a digger was working in the area of the damaged pipeline about three years ago.
"This has been confirmed following separate discussions with both the landowner's wife and a neighbour,'' a council spokesman said.
"To date, no one has been able to confirm the identity of the digger driver allegedly involved, however the council is now following a strong line of inquiry in that regard."
NRC's inquiries have included the scrutiny of publicly available satellite imagery taken over a period roughly three years ago.
But atmospheric conditions during that time prevented the NRC's satellite search striking oil.
"Regular cloud cover meant the site was not visible for much of the time and no useful imagery was able to be sourced," the spokesman said.
The council said the landowner's wife had been co-operative and had also spoken to representatives of NZ Refining and its insurers.
Efforts were now focused on ensuring a methodical, careful and thorough investigation into the cause of the leak.
During the 10-day repair in September, photographs of the broken pipe commissioned by Refining NZ revealed damage consistent with a digger blade having scraped it some time ago.
It has been suggested the compromised surface slowly degraded until the pipe ruptured.
Peat fields in the Ruakaka flats area have often been "mined" for swamp kauri, although there are strict rules against digging or planting over the clearly defined corridor where the pipeline is buried 2m deep.
Neighbours told the Northern Advocate they had not seen a digger working in that particular paddock for about three years.
The leak was discovered on September 14, with up to 70,000 litres of fuel having seeped into the ground.
The ensuing oil supply crisis resulted in flights being grounded and fears of a petrol shortage in Auckland.
Results of independent sampling in the vicinity have confirmed that bore water supplies have not been contaminated by petroleum hydrocarbons, the NRC said.
The council is continuing to work with Refining NZ on site remediation.
Legally, the NRC has until mid-March 2018 to lay potential charges relating to the spill; which technically and legally is classified "an unauthorised discharge of contaminants to land".
Refining NZ also said its energies remain focused on continuing to restore the site but was still waiting for metallurgic tests on the pipe and other results.
"We are co-operating with the NRC investigation, and will provide the results from the pipe tests carried out by the integrity expert when that report is complete."
As for internal or peer reviews of RNZ's systems and procedures, it was too early in the investigation process to answer questions about that, a spokesman said.