Is this down to the blanket change to 30kg on 15 September? Technically if the ticket is issued before 15th the old allowance applies, but people are being given 30kg if they ask. The ticket has the new allowance of 30kg, but the terms page still has the old allowance.

I don't think this has anything to do with the new baggage rule from 15 Sept.
Many years ago when I booked a Y ticket from SYD to PEK, the E-ticket I received stated 30KG, I have called CX before and the agent said it should be 20KG.
I don't know why this happened, but I think it has been like this for many years.
With my past experiences, I think you are safe as long as your bags do not weight too much over 30KG. AND BELOW 30KG IS DEFINITELY SAFE.

I wasn't 100% sure the new poster was referring to SYD flights.

Any single bag over 32kg and it doesn't matter what your allowance is...

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.

I'm referring to the health and safety rule that baggage handlers wont touch any single item over 32kg, in most places at least.