From the Australian guide on entry given to airlines and other travel industry companies/organizations/etc. (and I can't find the link now of course but I copied and pasted the following directly from it):

3.1 Australian passports
Australia issues Standard (blue), Official (green) and Diplomatic (red)
passports. Standard Australian passports are 32 pages. Frequent
traveller passports will have 64 pages. Most Australian adult
passports are valid for 10 years. In some circumstances, the validity
may be less than 10 years. Seniors’ (75 years and over) and
children’s passports are valid for five years.
Holders of Australian passports have a right to enter Australia and
do not require a visa.



As noted, Australia cannot deny you entry if you are an Australian citizen, whether your passport is expired, missing, you threw it into the ocean while drunk, etc. However, they can make your life miserable while attempting to figure out if you actually are a citizen. I would advise the friend on the expired passport to get it renewed ASAP, no matter time or cost. It will likely be cheaper/shorter than attempting to travel on the expired passport. And that assumes the airline even lets you board, as they aren't going to be as educated on the issues, and it only takes one agent unwilling to check you in before your flight leaves, even if on an incorrect basis or reasoning, to ruin your plans.