Survivors of a 2009 firestorm which killed 119 people in Australia Monday launched a class action against a Singapore power company over the disaster which lawyers described as "entirely preventable".

The Kilmore East blaze was the largest of the "Black Saturday" February 2009 wildfires in southern Victoria state that left 173 dead and razed more than 2,000 homes, the nation's worst natural disaster of modern times.

More than 10,000 people have joined the action against SP AusNet, which is majority owned by Singapore Power, over the Kilmore fire, which claimed 119 of the 173 lives lost.

Conservative estimates of the compensation payout are in the hundreds of millions of dollars, with a special courtroom purpose-built to accommodate the sheer volume of people involved in the case.

Lawyer for the plaintiffs Robert Richter told the Victorian Supreme Court the fire was started by a 43-year-old power line that collapsed in dry, hot and windy conditions, sparking a fast-moving and unpredictable blaze.

Richter said the firm had been warned of the catastrophic conditions in the days prior but took no steps to shore up the line, which was in a fire-prone area. The ensuing inferno razed more than 1,200 homes.

"This fire was entirely preventable," Richter said in his opening address to the court.

"SP might be right to say they did nothing to cause it ... but they did none of the things they should have done to prevent this damaged line from collapsing."

Richter said the power firm "knew the importance of scheduled replacement of old assets in high-risk areas" and it "might as well say that the invention of electricity brought about the chain of events which caused the fire".

The case is being led by Carol Matthews, whose son Sam, 22, was trapped in their home by the fast-moving fire and burned to death.

His final words to her in a frantic phone call, the sound of windows exploding in the background, was "Oh my God, there are flames everywhere," the court heard. Matthews wept in court as footage of the fire was played.

SP said an earlier lightning strike had been to blame and it intended to show that the damage "was so microscopic that no detection practice of the time, no matter how frequent or sophisticated, could have detected it".

"We understand and share the desire to determine the cause of this fire and to do all that can be done to avoid any events of this kind and magnitude from occurring again," it said in a statement about the case to its shareholders.

"SP AusNet remains confident that its practices and procedures did not cause or contribute to the Kilmore East bushfire."