Catastrophic fire conditions are the highest danger rating and signify extreme fire risk.
If fires start, they are fast-moving, unpredictable and very difficult to contain.
The Bureau of Meteorology said very hot and dry conditions, with fresh to strong, gusty north to north-westerly winds, were expected to develop over Victoria on Friday morning and persist through the afternoon.
A cooler and gusty west to south-westerly change is forecast over the south-west in the early afternoon, then extends east during the afternoon and evening.
Isolated thunderstorms are expected, mainly near and south of the ranges, with most activity likely in the afternoon and evening ahead of and near the change.
Little precipitation is likely with thunderstorms, bringing the risk of dry lightning and damaging wind gusts.
Victoria's emergency management commissioner, Tim Wiebusch, said the last time Victoria experienced catastrophic fire conditions was in November 2019, during the devastating Black Summer bushfire season.
"Catastrophic fire danger ratings mean that it will be unpredictable, uncontrollable and fast-moving fires that we will see," he said on Thursday.
Paramedics and first-responders have been relocated from branches in areas of catastrophic fire danger due to their proximity to bushland and limited road access.
"Crews will only be redeployed into these zones in the case of serious or life-threatening illness and only after a thorough risk assessment to consider the safety of our people," Ambulance Victoria said in a statement.
— Miles Proust