As a Canadian citizen flying from Australia to Bali, do I need to prove to the Australian airline at check in that I have booked a reservation out of Indonesia?
As of today, the Canadian site on Travel does not mention that it is required to have a return ticket. It does say immigration can require one while a passport must be valid for 6 months after arrival. That information was updated Jan 4, 2017, so it the most recent available now.
It was certainly possible to travel there without a return ticket in 2012 as a Canadian. Bali specifically in my case as I arrived on a flight from Hong Kong to Denpasar. Many of the documents quoted are irrelevant to this situation since they are either intended for the UK or are regarding visa-on-arrival which does not apply since Canadians are granted visa-free entry. Of course rules may have changed since 2012 (I did not keep a copy), so what was possible then may does not necessarily apply to what is possible today.
NOTE Redacted and rewrote most of this answer to reflect information available today. Check this answer's history if you are curious.
Contrary to Itai's answer, you are required to have a return or onward ticket. So the airline could prevent you from boarding without one. This is noted in Timatic, which is the reference the airline staff will check.
Immigration staff in Bali likely won't ask but they do have the right to check and deny entry if they wish.