A BLACK Saturday fire class action has been delayed because a new courtroom being won't be ready in time.

Victoria's largest civil court case, over Black Saturday's deadliest fire at Kilmore East, was due to start on January 29, but now won't begin until March 4.

Victorian Supreme Court Justice Jack Forrest said although work had started on the new courtroom, located in the William Cooper Justice Centre in central Melbourne, it would not be ready in time.

"It is now certain that the refurbished court will not be available for use by 29 January, 2013," he said.

Justice Forrest said inquiries had been made about temporarily using a court in the Federal Court, but it was not available, and no other suitable court could be found to accommodate the trial.

He said he appreciated the postponement could cause inconvenience to witnesses and parties involved in the case.

Earlier this month, Attorney-General Robert Clark announced the new courtroom was being refurbished and was due for completion in January, in time for the start of the Kilmore East trial.

The new courtroom would seat up to 40 lawyers at its bar tables and included public gallery seating for up to 100 people, he said.

An additional floor will host conference rooms, retreat and counselling rooms and media facilities.

Electricity company SP AusNet has vowed to "vigorously defend" the class action, which centres on claims relating to the inspection and maintenance of its assets.

The February 2009 Kilmore East bushfire, Black Saturday's deadliest, killed 119 people and destroyed more than 1000 homes in an inferno that devastated 125,383 hectares.

The trial is expected to run for six months.

The delay comes after a statement from Chief Justice Marilyn Warren that Victoria's highest court is in dire need of modernisation.

She said in the Supreme Court's annual report, tabled in parliament yesterday, that the court was struggling with its 19th-century environment and needed to modernise to meet the demands of mega trials and be fair to victims.