Seoul to Sydney (transit in Beijing and Shenzhen). Do I need a China visa?

Hi,
I'm an Australian citizen. On my return flight from Seoul to Sydney with China Southern Airlines, I have to transit in Beijing and then Shenzhen.
I've found out that the Beijing - Shenzhen segment is a domestic flight.
I have asked China Southern Airlines helpline, but they can't tell me if I need need a China visa or not.

I only have one week left before the travel date to obtain the visa, if I have too.
Can I make the connection on the airside of the airports and thus do not need to go through passport controls in China?

I would appreciate any answers.