Patrons at the Federal Hotel on Taupo Quay were left dodging sparks and flames after the main power fuse into the hotel exploded yesterday afternoon.
The Wanganui Fire Brigade sent two appliances to the scene but the fire had gone out before they arrived.
Owner Dave McKay said old wiring was to blame.
"It's lucky it went off now and not in the middle of the night," he said.
Co-owner Mary McKay said the whole side of the building moved. "It's a wonder the windows didn't blow in," she said.
Mark Wheeler, who was near the blast, said it sounded like a big bomb.
"I was on the phone to the fire brigade, but the bangs just got more powerful, and I didn't think I would make it out," Mr Wheeler said.
Wanganui Fire Senior Station Officer Bryce Coneybeer said the explosion was "quite spectacular".
"It blew itself to bits," he said.
The Power Board was last night trying to restore power to the pub.
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