An Australian nuclear physicist says engineers have not given a satisfactory answer as to why the cooling systems at two Japanese power plants are failing.

Japanese authorities say the nuclear reactors at the Fukushima and Onagawa power plants are having cooling problems because of power failures, and there are fears reactors at Fukushima may have already experienced partial meltdowns.

But the former chairman of the Australian Nuclear Science and Technology Organisation, Dr Ziggy Switkowski, says it should take more than a loss of power to cause problems with cooling.

"It strikes me as an unsatisfactory explanation," he said.

"These reactors have so many levels of back-up that the loss of primary power and then the loss of secondary power shouldn't have been enough to compromise the most critical element of a reactor, which is the cooling system.

"I think we're yet to learn all of the issues that are emerging or that were triggered by the earthquake."