Can I pay US or Aus $ for day-today expense while in China or I need to bring in Chinese currency in Beijing?

I’m afraid you cannot pay with US or Australian dollars for daily expense in China, and you can bring some Chinese currency in Beijing.

In major cities of China, you can convert your US or Australian dollars in the banks, such as Bunk of China. Generally speaking, the rate may be a little bit lower in China than in your own country.