A Black Saturday blaze that killed 119 people was entirely preventable and caused by the negligence of a power company that failed to maintain its power lines, a Victorian court has heard.

More than 10,000 people, led by plaintiff Carol Matthews, are suing energy provider SPI Electricity for negligence, claiming the collapse of a faulty power line ignited the Kilmore East/Kinglake bushfire on February 7, 2009.

At the opening of what is expected to be a 10-month trial in the Victorian Supreme Court, Robert Richter QC, for Mrs Matthews, said the case was not about proving who started the blaze because "SPI did".

He said the electricity provider had been warned of the catastrophic bushfire conditions forecast for Black Saturday - hot winds, searing temperatures and dry terrain - but took no action to maintain its lines in bushfire-prone areas.

"This fire was entirely preventable," Mr Richter told Justice Jack Forrest on Monday.

"With known and reasonable steps taken in time, SPI could and should have prevented it."

SPI claims it did everything that could have been expected of it.

It proposes that the power line failure was caused by a lightning strike.

Mr Richter said the failed power line was 43 years old and SPI should have expected that it would have been struck by lightning several times, causing fractures.

"They knew the importance of scheduled replacement of old assets in high-risk bushfire areas," he said.

"SPI might be right when they said we did nothing to cause the conductor to fail on that day.

"It's true, they did nothing.

"That's precisely why this action is being brought.

"They are liable for what they could and should have done before Black Saturday."

The class action group is also suing Utility Services Corporation Limited, which was contracted by SPI to maintain the line, and the Department of Sustainability and Environment (DSE) for allegedly failing to reduce fuel loads.

The CFA and Victoria Police are also facing allegations from Mrs Matthews that they failed to give appropriate warnings about the bushfire.

All the defendants deny the allegations and are fighting the claims.