All Australian visa, arrival and departure records are electronic, so passports are generally not stamped on arrival. (They are still often stamped on departure, even for Australian citizens, unless you use the automated gates. I'm not sure why.)
If you need proof of having been in Australia on certain dates, you can either request a stamp.on entry (and not use an automated gate, although this is not available to Indian citizens anyway), or request a Record of International Movements: http://japan.embassy.gov.au/tkyo/intlmovement.html
Note that for the latter, the application process is bureaucratic (certified copies etc) and processing time is several weeks, so it's not going to helpful if you have a short trip and need.to.prove it.immediately afterwards.
I'm reasonably sure that going back to the airport after arrival and requesting a stamp is not going to work, but let us know what happened if you do try it...