The budget airlines in Aus require you to be there 30 minutes before take-off.
I've missed a flight because of this; I was about five minutes late. I got on the next flight but the T&C clearly state they have no obligation to make it up in any way.
The only time where you might get away with arriving 20 minutes before take-off is if there's no security line and your gate's right near the entry to the departures lounge.