Expert witness are giving evidence at the Dunalley bushfires negligence suit against a Forcett landowner.
Melissa Barrett is being sued for allegedly failing to properly extinguish a campfire in a tree stump before the 2013 firestorm destroyed hundreds of buildings.
Tasmania Fire Service investigator Barry Bones took the stand on Tuesday in the Supreme Court in Hobart.
Mr Bones testified the roots of a smouldering tree stump on Ms Barrett's property kept burning underground before igniting vegetation on that January day.
Ms Barrett's lawyers have tendered reports which have cast doubt on the stump findings, claiming the blaze could have started from glass focusing sunlight, a discarded cigarette, or even a deliberate act.
But Mr Bones testified under oath none of those were adequate explanations.
"There was no evidence or information to collaborate this [deliberately lit] possibility; there was no person or persons around the area of ignition prior to the fire starting, so we have no reason to believe that is the case," he said.
"Conditions have to be quite specific for a cigarette to set alight vegetation; as you can see in that photo there, the cigarette butt is actually sitting on unburnt vegetation."