Last year's Fukushima nuclear accident was a man-made disaster caused by Japan's culture of "reflexive obedience" and not just the tsunami that hit the plant, a damning parliamentary report said Thursday.

Ingrained collusion between plant operator Tokyo Electric Power, the government and regulators, combined with a lack of any effective oversight led directly to the worst nuclear accident in a generation, the report said.

"They effectively betrayed the nation's right to be safe from nuclear accidents. Therefore, we conclude that the accident was clearly 'man-made'," said the Fukushima Nuclear Accident Independent Investigation Commission.

"We believe that the root causes were the organisational and regulatory systems that supported faulty rationales for decisions and actions," it said.

The probe is the third of its kind in Japan since the huge tsunami of March 2011 crashed into the Fukushima Daiichi nuclear power station.

Reactors went into meltdown, sending clouds of radiation over a wide area, forcing tens of thousands of people from their homes, some possibly forever.

An earlier report by plant operator Tokyo Electric Power (TEPCO) had all but cleared the huge utility, saying the size of the earthquake and tsunami was beyond all expectations and could not reasonably have been foreseen.

But an independent group of scholars and journalists, who reported their findings in February, said TEPCO could and should have done more.

It also said that had the company had its way, its staff would have been evacuated from the crippled plant and the catastrophe could have spiralled even further out of control.

In his straight-talking preface to the more than 600-page report, panel chairman Kiyoshi Kurokawa said difficult lessons that go to the heart of the national character had to be learned from the catastrophe.

"What must be admitted -- very painfully -- is that this was a disaster 'Made in Japan'," he wrote.

"Its fundamental causes are to be found in the ingrained conventions of Japanese culture: our reflexive obedience; our reluctance to question authority; our devotion to 'sticking with the programme'; our groupism and our insularity.

"Had other Japanese been in the shoes of those who bear responsibility for this accident, the result may well have been the same."

Kurokawa said in the wake of the 1970s oil shocks, nuclear power had been embraced with an unquestioning singlemindedness that left the industry "immune to scrutiny by civil society."

"Its regulation was entrusted to the same government bureaucracy responsible for its promotion.... A tightly-knit elite with enormous financial resources had diminishing regard for anything 'not invented here'.

"This conceit was reinforced by the collective mindset of the Japanese bureaucracy, by which the first duty of any individual is to defend the interests of his organisation.

"Carried to an extreme, this led bureaucrats to put organisational interests ahead of their paramount duty to protect public safety."

Kurokawa said this way of thinking meant organisations wilfully ignored the lessons they should have learned from previous nuclear disasters and the covering up of small-scale accidents became accepted practice.

"It was this mindset that led to the disaster at Fukushima," he said.

"The consequences of negligence at Fukushima stand out as catastrophic, but the mindset that supported it can be found across Japan. In recognising that fact, each of us should reflect on our responsibility as individuals in a democratic society."

The findings call for further investigation into the impact of the 9.0 magnitude earthquake -- as opposed to the towering tsunami -- on the reactors at Fukushima.

"As for direct cause of the accident, the commission reached the conclusion that we cannot definitely say any devices that were important for safety were not damaged by the earthquake," it said.

"We cannot rule out the possibility that a small-scale LOCA (loss-of-coolant accident) occurred at the reactor No. 1 in particular."

Although many scientists and activists have questioned the dominant narrative that cooling systems were knocked out by the rising waters, the government and TEPCO have been unwilling to say the reactors were damaged by the initial earthquake.

Tectonically-volatile Japan has a network of nuclear reactors that, until Fukushima, had supplied around a third of the nation's electricity.

The nuclear industry has long boasted of its many safeguards against earthquakes, but much recent public opposition to atomic power has focused on the vulnerability of plants, especially those that sit near seismic faults.

TEPCO spokesman Junichi Matsumoto said the company would be carefully reading the report before responding fully.