A radioactive water leak has halted plans to re-start a reactor at a nuclear power plant in western Japan, which would have been the fourth to come online after a nationwide shutdown, its operator said Sunday.

Kansai Electric Power said some 34 litres (8.8 gallons) of cooling water containing radioactive substances leaked out from the reactor at its Takahama plant 380 kilometres (236 miles) west of Tokyo.

"Resumption procedures related to the incident have been suspended as we are still investigating the cause," a company spokesman said, adding that there was no impact on the environment outside the plant.

The government and utility firms have been pushing to get reactors back in operation nearly five years after a huge earthquake and tsunami caused a disastrous meltdown at the Fukushima nuclear power plant.

The accident forced all of Japan's dozens of reactors offline for about two years in the face of public worries over the safety of nuclear technology and fears about radiation exposure.

Last month, another reactor at Kansai Electric Power's Takahama plant was switched on, but the accident stalls plans to bring the next one online which have already met with stiff opposition from local residents.

The Fukui District Court in December overturned an injunction preventing a re-start of the two reactors which had been won by residents, who argued it was not proven to be safe despite a green light from the national Nuclear Regulation Authority.

Two reactors in the southern prefecture of Kagoshima, operated by Kyushu Electric Power, restarted in August and October 2015, ending the two-year hiatus in nuclear power generation.

But many Japanese remain wary and thousands of former residents have refused to return to areas hit by the Fukushima meltdown over fears of radiation exposure.