The man in charge of the clean-up at Japan's Fukushima nuclear plant says growing stores of contaminated water from the site will eventually have to be dumped into the sea.

In an exclusive interview with the ABC, the chairman of the Fukushima Monitoring Committee, Dale Klein, has also admitted there are likely to be more blunders and slip-ups at the plant in the months and years to come.

"I think the best word to use with Fukushima is challenging," the former chief nuclear watchdog in the US said.

A worker at the plant has told the ABC there are no back-up systems at Fukushima. ( AFP )

Since the 2011 earthquake and ensuing tsunami sparked the nuclear disaster, TEPCO has been pouring millions of litres of water onto Fukushima's reactors to try and keep them cool.

That radioactive water is now being stored in tanks at the site but already thousands of litres have leaked into the Pacific Ocean.

Mr Klein says the biggest challenge for TEPCO is dealing with these ever-growing volumes of contaminated water being stored at Fukushima.

He believes that after the water is treated and stripped of most radioactive elements, it will be safe to dump into the Pacific.

"At the end of the day, when the water is discharged, it will be released in a way that it's diluted," he said.

"So there's no risk to public health and safety. But it's an emotional issue."

Rat 'chewed through' Fukushima wiring

It is not just contaminated water causing problems at Fukushima.

Hastily and shoddily built systems at the nuclear plant have also triggered some anxious moments.

The ABC has spoken to one man who worked at the Fukushima site until he reached his maximum radiation exposure limit.

He still works for TEPCO, and only agreed to speak to the ABC on the condition of anonymity.

"I was at the plant earlier this year when a rat chewed through some wiring and knocked out power to several used fuel pools," he said.

"I thought, 'how could one rat cause such a big problem?' There were no back-up systems."

Mr Klein has told the ABC he believes there will be more dramas at Fukushima.

"I think we will see more of those. When you look at that site, it's massive," he said.

"It's a big site and it's not unusual to have other things like that."