There was one time my bag weighted around 35KG and it went thru without any problem (departed from SYD), the check-in staff didn't say a single word about it, but I wouldn't refer that as a normal case that would happen every time.

When I was a AM member, I usually pack 25-30KG and the bag went thru every single time without any problem so I would say below 30KG would be safe (all SYD departure). But of couse if you don't want to risk it, then probably follow the 20KG rule.