Australians numbering 145 are still unaccounted for in the Japan earthquake as rescue operations are hampered by new aftershocks with 5.7-magnitudehit the city of Tokyo this morning.

The US Geologiocal Survey said that at 5:43 AM a temblor hit with a 5.7 magnitude, while a 6.1 magnitude occurred last night southwest of the capital.

Japanese authorities and foreign group rescuers could not cope with the turn of events and this afternoon, the Tokyo Electric Power Company has suspended the operations near the severely damage Fukushima Daiichi nuclear plant as workers exposure to increading radiation levels have been quite alarming.

In a news briefing this afternoon by Prime Minister Julia Gillard, she said that the 145 Australian nationals were not yet accounted for since the Friday quake occurred.

She noted that two of the members of the rescue team tested positive for some low-levels of radiation.

"The clear advice to me is that these two personnel are safe and well," she said.

Ms Gillard said the International Atomic Energy Agency had advised her that health risks, from exposure to the radiation, was low to negligible.

''Our nuclear experts advise that there is a small chance of contamination at very low levels for Australians who were in the Fukushima area,'' she said.

The two were also on board the chopper circling the area of Fukushima, but were forced to land as the chopper engine froze due to build up of snow in its rotors.

The American helicopter carrying four Australians and New Zealand search and rescue experts had been forced to land at Fukushima Airport after a build-up of ice on the aircraft's rotors.