An anti-nuclear candidate pulled off a surprise victory in a local Japanese election at the weekend, hurting Tokyo's already struggling bid to restart shuttered reactors more than five years after Fukushima.

First-time politician Ryuichi Yoneyama, 49, had campaigned on a pledge to stop the refiring of the Kashiwazaki-Kariwa power station, the world's biggest nuclear plant, about 200 kilometres (130 miles) northwest of Tokyo.

Yoneyama won the race late Sunday to become Niigata prefecture's new governor, in the latest challenge for the energy policy of Prime Minister Shinzo Abe, who called the election results "extremely regrettable".

Shares in Fukushima operator Tokyo Electric Power -- which also runs Kashiwazaki-Kariwa -- tumbled nearly eight percent to 385 yen in Tokyo on the election news.

There are seven reactors across the 4.2 million square metre (45 million square feet) Kashiwazaki-Kariwa site.

"I can't approve (restarting reactors) in the current situation where the lives of residents can't be protected" from an accident, Yoneyama told reporters on Monday.

His victory came after voters in southern Kagoshima prefecture voted in a new anti-nuclear governor in July.

Yoneyama, a doctor and lawyer who ran as an independent but was supported by left-leaning opposition parties, beat a ruling bloc-supported candidate with 528,455 votes to his opponent's 465,044.

The unexpected results put Abe in a tricky position.

The central government can overrule a governor's opposition to restarting nuclear reactors. But Abe has promised to win approval from local communities before approving restarts under stricter post-Fukushima safety rules.

Dozens of reactors across Japan were switched off in the wake of the March 2011 Fukushima accident, the worst nuclear disaster in a generation, and there are just two currently operating.

The catastrophe forced resource-poor Japan to turn to expensive fossil fuels to plug its energy gap.

But fears about the safety of nuclear power and radiation exposure linger, challenging a push by Abe and utility companies to switch the country's stable of reactors back on.

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