An explosion has shut down half of Victoria's largest gas-fired power station and it could be months before it is back to full capacity.

But energy authorities say there is no threat to the state's supplies as a result of the early-morning blast.

Fire crews were called to Mortlake Power Station just after 6am on Monday for what Origin Energy said was an electrical outage.

Mortlake Power Station is operating at half capacity following an explosion. Credit:Photo: Leanne Pickett

She said Origin was still determining how long the outage would last, but CFA commander Paul Marshall told 3AW that he believed it could be offline for months.