A digger almost certainly broke the vital Ruakaka-Auckland fuel pipeline, but it could have happened anytime in the past few years.
Refining New Zealand (RNZ) spokesman Greg McNeill said experts, including insurance assessors, had examined the corroded, leaking section of pipe in-ground and after removal and were certain scrape marks on it had been caused by a digger.
''It's six feet deep and the signs, and logic, indicate a digger,'' he said of the buried pipe.
The Ministry of Business, Innovation and Employment had not confirmed when an inquiry might begin into what caused the damage, which has disrupted air travel and could lead to panic-buying of already-vulnerable petrol stocks.
However RNZ said there was no sign of recent digging or ground disturbance at the site where between 70,000 and 80,000 litres of aviation gas had leaked.
The leak was discovered last Thursday afternoon after instruments showed a drop in pressure in the volume going through the pipeline and an electronic probe pinpointed where the problem was.
About 30 staff at working day and night to repair the pipe.
Mr McNeill said investigations would continue but ''our focus now is getting that pipeline back".
Any higher level inquiries would likely be held at Government level and by Northland Regional Council, he said.
The wedge of land between the Ruakaka coast and State Highway 1, was once a large peat swamp. Much of it is drained farmland now, but swamp kauri has been excavated and reburied in the past, but not for several years. There is no proof that's linked to the leak.
A neighbour said he had not seen a digger on the farmland the pipeline passes under for ''a long time''.
Repair work on the pipe is continuing round the clock and it is still expected to be repaired by Sunday, though it could take until Tuesday before it is fully operational again.