Spontaneous combustion of an oily rag could have started a fire which razed a building and threatened a stand of native trees.
The June 14 blaze, which destroyed a 150sq m shed containing machinery, tools and timber, has prompted a warning to carefully dispose of rags used with vegetable-based oils.
Firefighters could do nothing to save the shed but did stop the fire spreading into overhanging trees at a remote waterfront property on Waikino Rd, east of Kawakawa.
Initially the cause was put down to a recharging nail gun. Fires in similar lithium batteries have bedevilled the new Boeing 787 Dreamliner aircraft.
However, an insurance company fire investigator has pinned the blame on an oily rag.
The home owner had been polishing furniture with linseed oil the previous day, and afterwards threw the used rag onto a heap with other rags.
Investigator David Noble said linseed oil, like other vegetable oils, contained a natural oxidising agent. Under the right conditions a cotton rag containing oil would heat up until it started smouldering and eventually ignite.
It was important to dispose of such rags properly or leave them outside to dry thoroughly. They should not be put in a paper rubbish sack, or in a bin or on a pile with other combustibles.
Linseed oil bottles carried a warning but it was often in small print.
A similar phenomenon was responsible for most laundry fires.
Tea towels not put through a hot wash could gradually build up cooking oils until they spontaneously combusted, either in the drier or when stacked with other towels.
Such fires were ''not uncommon'', Mr Noble said.
Fire Service investigator Craig Bain agreed that rags used with linseed oil should be hung outside on a clothesline until the oil had completely evaporated. They should not be screwed up and left in a pile.
The owner of the Waikino Rd property, who did not want to be named, wanted the danger brought to public attention.
''It could save a lot of people a lot of grief,'' he said.
Kerikeri deputy fire chief Greg Imms said the brigade had had a number of call-outs sparked by oily rags over the years, and one due to combusting tea towels at a Kerikeri laundromat.