A misguided faith in the complete safety of atomic power was a key factor in Japan's 2011 Fukushima accident, the UN nuclear watchdog said in its most comprehensive report on the disaster.

The International Atomic Energy Agency (IAEA) pointed to numerous failings, including unclear responsibilities among regulators along with weaknesses in plant design and in disaster-preparedness.

But possibly the biggest factor was the "widespread assumption in Japan that its nuclear power plants were so safe that an accident of this magnitude was simply unthinkable", IAEA director general Yukiya Amano said in the report of more than 1,200 pages.

A quake-sparked tsunami swamped cooling systems and triggered reactor meltdowns at the Fukushima plant in March 2011.

Radiation spread over a wide area and forced tens of thousands from their homes -- many of whom will likely never return -- in the worst nuclear accident since Chernobyl in 1986.

The IAEA report, published late Monday, criticised safety assumptions by the nuclear plant operators that were not challenged by regulators or the government.

As a result, the quake-prone nation "was not sufficiently prepared for a severe nuclear accident".

Operators assumed there "would never be a loss of all electrical power at a nuclear power plant for more than a short period" and did not consider "the possibility of several reactors at the same facility suffering a crisis at the same time", it added.

"Since the accident, Japan has reformed its regulatory system to better meet international standards. It gave regulators clearer responsibilities and greater authority," Amano said.

"There can be no grounds for complacency about nuclear safety in any country. Some of the factors that contributed to the Fukushima Daiichi accident were not unique to Japan."

Anti-nuclear sentiment still runs high in Japan, which last month began restarting its atomic power programme after a shutdown triggered by Fukushima.

Utility Kyushu Electric Power turned on a reactor at Sendai, about 1,000 kilometres (620 miles) southwest of Tokyo.

Commercial operations for the 31-year-old reactor -- operating under tougher post-Fukushima safety rules -- will begin early this month, as the government pushes to return to a cheaper energy source than fossil fuels.

The resource-poor nation, which once relied on nuclear power for a quarter of its electricity, restarted two reactors temporarily to feed its needs after Fukushima. But they both went offline by September 2013, making Japan completely nuclear-free for about two years.

The government wants nuclear power to generate up to 22 percent of Japan's electricity needs by 2030, a lower percentage than before Fukushima.