The federal government has urged Australians within 80km of Japan's stricken Fukushima nuclear power plant to evacuate, amid mounting concerns of a full-scale meltdown.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) said Australians should observe an exclusion zone much bigger than the 20km one imposed by Japanese authorities.

"The Department of Health and Ageing and ARPANSA believe this is prudent advice as a precautionary measure," it said in a statement on Thursday.

"While information on radiation levels and wind direction appear unchanged, the situation at the power plant is not stable and it is unclear what will evolve."

ARPANSA's advice came shortly after the US made a similar recommendation to its citizens.