Former Japanese prime minister Naoto Kan, who was in office during the Fukushima nuclear meltdowns, has told the ABC he believes the plant's operator has been hiding key evidence.

For months TEPCO has resisted pressure to release critical recordings, arguing they are in-house material and to release them would compromise the privacy of those on the tapes.

The company now says it will bow to months of pressure from Mr Kan and the government and release the many hours of teleconference video taken in the days after last year's meltdowns.

But it has confirmed crucial audio of a heated exchange with Mr Kan is missing, claiming its hard drive was full.

'Extremely strange': former Japanese PM Naoto Kan says TEPCO has something to hide ( Reuters )

The recordings are a window into the decisions, dramas and dangers at the heart of the world's worst nuclear crisis since Chernobyl, and Mr Kan says the withheld information is "crucial".

"This is extremely important material which is needed to investigate the cause of this accident," Mr Kan told the ABC in an exclusive interview.

"It's like the black box flight recorder on an aeroplane. The black box is crucial to understanding how an accident happened. So too are these recordings."

Lawyer Hiroyuki Kawai has been fighting for the release of the TEPCO tapes. He believes they will help solve some abiding mysteries at the heart of the disaster.

"TEPCO executives purposely delayed pumping seawater into the damaged reactors," Mr Kawai said.

"The tapes will prove they did this because they knew pumping in saltwater would force them to decommission the reactors."

Detailed records

Another mystery the recordings could help solve is who is telling the truth about TEPCO's early handling of the crisis, specifically whether the company wanted to withdraw its workers from the crippled plant, as Mr Kan has alleged.

"The most contentious issue is whether or not TEPCO was planning to abandon the Fukushima plant," he said.

"The audio and video will make it clear what discussions TEPCO was having about withdrawing."

The recordings may also clear up persistent questions about Mr Kan's handling of the crisis, specifically allegations he meddled at key moments, effectively delaying TEPCO's response to the disaster.

But the crucial recording of Mr Kan addressing the company's 200 executives and its president at the start of the crisis has no audio, which the company blames on a lack of hard drive space.

Mr Kan has described the missing audio of his speech as "extremely strange".

"The speech was filmed and broadcast to all TEPCO sites. Surely they recorded the sound at one of those sites.

"It would appear the company is trying to hide something inconvenient."

TEPCO now has to decide when to release the teleconference recordings and how it will release them.

Mr Kawai believes the release of the tapes is crucial if Japan is to both understand the causes of the nuclear meltdowns and to learn from them.

"The Fukushima nuclear accident is ongoing. It's not been resolved yet," Mr Kawai said.

"The TEPCO recordings are a common asset of the Japanese people and not for a private company like TEPCO to hide."

The revelations come as tens of thousands of people rally in Tokyo to demand an end to nuclear power, the latest in a series of protests since the Fukushima disaster.