Eligibility for 72-hour visa-free entry

My sister and I are visiting Beijing in November this year, arriving evening Tuesday 20th November and departing onto Hong Kong Friday 23rd November. We will be arriving from Sydney Australia, transiting Hong Kong International Airport, then going back to Hong Kong for 12 days before returning to Australia.
Can you advise if we will be able to enter Beijing on the 72 hour visa-free arrangement or will we require visas.
Thank you, Susan Munro