According to the Australian government's website (http://www.citizenship.gov.au/current/travel/
), "[a]s an Australian citizen you must always leave and enter Australia on an Australian passport." This makes it sound like a legal requirement, but I have no idea precisely what Australian law says on this point.
The FAQs on the Australian High Commission website in the UK note some of the inconveniences that might be experienced by an Australian citizen arriving without an Australian passport (http://www.uk.embassy.gov.au/lhlh/PptsFAQs.html;
see Q9 and Q10). Here it doesn't say that it would actually be unlawful for an Australian to enter the country on a foreign passport, and in such a situation, the citizen would have to be admitted once his/her identity was verified, but he/she might face delays at the port of entry and possibly even difficulties in being allowed by an airline to board his/her flight. The risk of such difficulties would be much diminished, I imagine, if the dual citizen in question was traveling for a visit on a round-trip ticket with all other paperwork, including a valid foreign passport, in order, but there's still a risk that one might encounter a few hassles.
The High Commission's FAQs note too that "an Australian citizen cannot be granted a visa for Australia." Don't even European citizens need some kind of advance electronic travel authorisation before boarding a flight to Australia? If such an authorisation cannot be granted to an Australian, that might complicate matters, too.
Based on all of the above, it sounds like it would be worth getting a passport in advance of travel if at all possible, but if you can't, and you wanted to try presenting yourself as a Swiss citizen visiting Australia (which you are, after all), you might just want to see if the Australian eVisitor application works for you (I don't know if it asks about Australian nationality as part of the application process).