I am a Indian passport holder and was going back to India when my immigration card was not checked.
How do I get my passport stamped?
Australia does not routinely stamp passports on exit. So it is perfectly normal that you left without getting an exit stamp.
They do maintain electronic entry-exit records, so they will know whether you left on time and will not hold the lack of a stamp against you.
If you're particularly concerned or think your exit may not have been recorded properly (perhaps if you entered on one passport and left on another; I'm not sure if this situation is fully accounted for), you could simply retain proof of your exit, such as your boarding pass back to India, just in case.