It is not a requirement of the Australian government that you have proof of onward or return journeys altho a lack of them may incur some extra 'discussions' at point of entry.
Normally they want to make sure you have the funds to remain in the country for the time allotted by your visa as well as leave. Sometimes they like to see a 'connection' to your country of residence.
Of course airline regs can negate this.
You will need a visa to enter Australia. USA passport holders travelling for tourism usually get an ETA best purchased from the Australian Immigration Dept web site.
Christmas is our long summer break. Many Australians travel overseas from mid Dec to begin Feb (altho the falling $A may slow this down a little) so be aware that flights can be pretty full.....coming and going.