Geoffroygd,
If you have a valid visa ie, the visa that back packers get for a working holiday here in Australia then you are free to work where ever you wish. I would suggest you get a criminal background check done in your home contry now so you can present it to the Oz authorities when you come here. Apparently not being an Oz citizen is no impediment, as long as you are of good charicter. I recently worked with a Candaian guy in sydney who did just what you want to, and was even able to get an ASIC.
The harder part will be getting a company to put you on, knowing you will only be there for 3 months.
I dont know who does handling in Melbourne as I'm in Sydney. Try Aerocare (
www.aerocare.com.au), they do some ramp handling in Melbourne.
Feel free to PM me if you have any more questions.
Good Luck