I am an Australian Citizen, how can I get transit visa to stay 2 Nights in Guangzhou

Punya, no, you cannot get VOA in Guangzhou, but if you fly like Australia-Guangzhou (2 nights)-USA, you will be able to enjoy the free transit in Guangzhou, and then no visa will be required.

Uther is correct. If you are arriving in Guangzhou from a foreign city and are departing directly to another foreign city (or Hong Kong) you qualify for 72 hour free transit. You cannot apply for a longer stay visa on arrival.
Make sure you have evidence of hotel booking or other accommodation in Guangzhou & can show departure ticket to USA.
They are very strict & if you don't have the info they will detain you at the airport and put you back on a plane.... I know from experience, but have been back 3 times successfully. Great place to visit.