If I am staying only overnight, going through Beijing to Ulaanbataar, Mongolia

Can I stay inside the airport, without leaving for 24 hours - ie. are there rooms etc., one can rent? Do I need a visa, as an Australian citizen? If so, can I get one on arrival or must I go through the Chinese Embassy in Tokyo (I live in Japan), in order to leave the airport for any period of time, no matter how short?
Thanks in Advance.