However, people in a vast area of Balochistan were still without electricity, Interior Ministry officials in Islamabad said, adding: "It could take two weeks to fully restore the damaged transmission lines."
A shadowy group called the Balochistan Liberation Army said late on Tuesday it had carried out the attack in Sibi.
It was the third attack in two weeks amid a surge in violence blamed on suspected nationalist tribesmen, who want more political rights plus a bigger cut of earnings from the region's ample natural resources.
Last month, two other power transmission lines were blown up by insurgents and officials said the Sibi transmission tower was linked to the last major power line in the rugged province.
"This was the last power supply line after two power transmission lines were blown up earlier and now we have no means to supply power," an official from the government's Water and Power Development Authority said late on Tuesday.
Several rockets were fired, which blasted four electricity pylons before a bomb explosion ripped apart the main transformer, the official said.
The Balochistan Liberation Army also claimed responsibility for a blast near a railway line in Quetta on Tuesday in which three people were wounded.
Police said they believed the man who planted the bomb at the railway line was killed later when an explosive in his scooter exploded prematurely. Two other people were wounded.
The group has claimed many recent attacks, including rocket strikes on Pakistan's largest gas field at Sui, which left eight people dead, and a bicycle bomb in Quetta that killed 10 people in December.