MARK COLVIN: But first an exclusive insight into one of this year's biggest stories - the Fukushima meltdowns.

Kenichi Matsumoto is the ultimate insider.

As special advisor to Japan's prime minister and cabinet he witnessed both the government's and the plant operator's responses to the worst nuclear accident in a quarter of a century.

And when it comes to the meltdowns, Professor Matsumoto paints a picture of cover-ups, incompetence and communication breakdown.

He confirms that the operator of Fukushima - TEPCO - wanted to abandon the stricken plant and that the prime minister at the time - Naoto Kan - contemplated evacuating tens of millions of people from in and around Tokyo.

Professor Matsumoto also accuses the Japanese leadership of knowing months ago that areas around the nuclear plant would not be habitable for decades.

North Asia correspondent Mark Willacy reports from Tokyo.

MARK WILLACY: He's been described as the prime minister's 'brains trust' but Kenichi Matsumoto isn't a nuclear physicist or a scientific genius. The history professor and author was a special advisor to the Japanese cabinet when a tsunami slammed into the Fukushima nuclear plant.

So he would become a witness to history and he's given the ABC an ultimate insider's account of what happened in the hours and days after March 11, as three of the Fukushima reactors bubbled towards meltdown. And he's damning of the plant's operator - TEPCO.

KENICHI MATSUMOTO (translated): First TEPCO did not convey accurate information about the accident to the Prime Minister. It tried to make the disaster look small. Then TEPCO's headquarters wanted to evacuate the nuclear plant, but the chief of the facility vowed not to leave. So prime minister Kan was outraged because he wasn't getting proper information or the truth.

MARK WILLACY: This lack of clear and accurate information was feeding panic, both in communities around the Fukushima plant, and around the cabinet table in Tokyo.

In the end TEPCO was ordered to keep its people at the plant and to start feeding the government more information.

Special advisor Kenichi Matsumoto reveals that the prime minster at the time, Naoto Kan, was considering evacuating 30 million people, after being briefed on a worst-case scenario.

KENICHI MATSUMOTO (translated): It's true that the Prime Minister said we might have to evacuate people from Tokyo. There was no clue about the amount of radiation coming from the Fukushima plant, or if it was spreading over 100 or 200 kilometres. If that was the case, Tokyo would be in danger. And Prime Minister Kan actually said that eastern Japan might not be able to keep functioning; that it might collapse.

MARK WILLACY: In the end, talk of tens of millions being evacuated was dismissed, with fears it could cause mass panic and chaos, worse than the nuclear crisis itself.

But at the time what was collapsing, or more accurately, melting, were the fuel rods in reactors 1, 2, and 3, after they were fully or partially exposed. In less than 24 hours, the number 1 reactor core had melted and burnt a hole through the pressure vessel. It wasn't until three months later that the Japanese government confirmed that the outer containment vessel had also been breached.

But special advisor to the cabinet, Kenichi Matsumoto isn't just critical of TEPCO's handling of the nuclear crisis, he's also scathing of the then prime minister and his former boss Naoto Kan.

KENICHI MATSUMOTO (translated): I don't think he handled it well. Because it was such a terrible accident, information should have been shared with the whole cabinet. But it wasn't. The information stopped with Mr Kan who handled it alone. So the cabinet was isolated and wasn't able to formulate its advice properly.

MARK WILLACY: Mr Kan has since resigned and Kenichi Matsumoto has also left his post as special advisor to the cabinet. What remains are 80,000 people displaced by the nuclear disaster. They're now in their seventh month living in shelters or temporary housing and many are desperate to know if they can ever return to their homes.

Kenichi Matsumoto says the government has known for months that thousands will not be able to return.

KENICHI MATSUMOTO (translated): The cabinet knew right after the disaster that some people would not be able to live in their communities for 10 to 20 years, especially those a few kilometres from the plant. The government should have conveyed the truth to the evacuees. But it felt scared; it feared telling the truth to the people.

MARK WILLACY: Kenichi Matsumoto has now left politics for the more sedate world of academia, returning to his history post at a university outside of Tokyo.

But he's still determined to write the history of the Fukushima nuclear crisis from his unique perspective from the inside.

This is Mark Willacy in Tokyo for PM.

MARK COLVIN: We approached the former prime minister Naoto Kan for a response but received no reply. A spokesman for TEPCO told us the company never tried to downplay information about the nuclear disaster, but acknowledged that there were mistakes made and some confusion at the start of the crisis.