Qantas Airways Ltd. is maintaining its original flight capacity between Sydney and Tokyo but is resting flight crews in Hong Kong as a potential nuclear crisis and earthquake aftershocks continue to rock Japan.

Australia's biggest carrier said it is still operating a daily return service between Sydney and Narita airport and three-times weekly services between Perth and Narita.

Seats are available on all sectors, a Qantas spokesman said.

Flight crews are resting overnight in Hong Kong given that Tokyo is "not a conducive environment for that, particularly in terms of aftershocks being experienced", the spokesman said.

Flight paths over Japan haven't been diverted and Qantas isn't testing radiation levels on its flights, but the situation is being very closely monitored, the spokesman said.