Questions have been raised about the cause of the Kilmore East bushfire on the eve of a $1 billion class action.

The Victorian 2009 Bushfires Royal Commission found a jammed end connection on a power line sparked the Black Saturday blaze in 2009.

But experts and the victims’ legal team now agree there was another cause, according to media reports and a statement by one defendant, electricity company SP AusNet.

An earlier lightning strike led to the cracks that made the power line susceptible to wind stress and caused its failure, experts now say.

Lawyers Maurice Blackburn, for the victims, will argue the damaged line should have been replaced years earlier and the power company failed to undertake an appropriate asset inspection.

SP AusNet has told the Australian Securities Exchange the lightning damage was so microscopic no detection technique could have found it, the company.

The class action, beginning today, involves about 10,000 claimants seeking damages from defendants including SP AusNet, emergency services and the Victorian Government’s Department of Sustainability and Environment.