More than five thousand homes and businesses across up to 40 towns were plunged into darkness at 10.35pm after the single nocturnal creature caused a fault at the Stawell Zone Substation.

At the height of the mass blackout about 3,422 Stawell customers were left in the dark with few surrounding towns spared a loss of power.

Scores of towns across the region including Great Western, Landsborough, Navarre, Concongella, Joel Joel, Marnoo, Callawadda, Campbells Bridge, Greens Creek, Bulgana, Crowlands, Rhymney, Moyston, Armstrong, Black Range, Pomonal, Halls Gap, Glenorchy, Deep Lead, Dadswells Bridge and Mokepilly were impacted by the power outage which lasted for close to two hours.

Powercor spokesperson, Mary Tait said how the possum made it onto a transformer at the substation was yet to be determined.

However, the cause became fairly obvious to Powercor workers who first responded to the outage call.

"The possum was found to be deceased on the transformer, so it became obvious that it was the cause of the initial fault," she said.

"We aren't exactly sure how it got there, but as we all know wildlife can be pretty ingenious at times."

Power was steadily restored to the thousands of premises affected at 12.29am.

Ms Tait said due to the reasonably large nature of the power outage crews returned to the site yesterday.

"We apologise for any inconvenience caused as a result of the power outage, our crews worked incredibly hard to ensure customers were reconnected as soon as possible," she said.