I've flown with Qantas and this is what I have found. They say the max weight is 20kg (as an example), but the max weight on the scales is 35kg (I think). So you can pretty much have luggage anywhere in that range and you will be ok, they'll just put a bright orange sticker or tag on your luggage to say heavy. If you are over the 35kg mark they normally get you to split your luggage up. This is more for OH&S reasons than anything else as the luggage handlers are not allowed to lift anything over 35kg. I've been over the 35kg mark and had to split my luggage up, but didn't get done for excess baggage.
HTH,
Mel