can I apply for a student visa X2 in Japan? I am Australian and have already spent 6 months in China

According to Chinese embassy in Japan, you can submit your entry records and other required documents to apply. But I would suggest you confirm this with the embassy.
Tel: 03-3403-5633、03-3403-5232、03-3403-3419、03-3403-0924

If you can't get a confirmed answer, I think you'd better get back to Australia to apply. This is the safest way.