I have heard they keep visa overstayers at Villawood detention centre-not a nice place!! As great as Australia is-you dont want to end up been sent there-even if for just a short time until they can deport you. I also suspect that what jasonh said about it maybe affecting u in other countries may be true. Im sure when I applied for a usa work visa I was asked if I had ever been deported from another country.
If I was u I would contact immigration, explain your situation and see what happens.
You honestly don't want this to bite you in the future. Like it has been said, the IMMI are helpful if you approach them and let them know what you are doing. A black stamp in your passport is something you don't want to have.
You won't be tossed into a detention centre unless you overstay and are caught somewhere.
If you are leaving the country and have a flight booked they will simply tell you that you overstayed, that you can't come back and ban you for 3-5yrs...thats it....5 min chat.
So if you don't plan on coming back...go for it!