Its all about $.
They could sell J all the way from AKL to LHR, but then, the service from MEL/SYD to LHR is in F/subclass of F, so its all to cover their $.
As we know, there is no F on AKL - MEL/SYD, but they can sell the tix as F just because the continuation of the service is in F from Aust - LHR.
I have not had a look at the difference (separate PNR) in price if AKL - MEL/SYD fares, is in J and then MEL/SYD - LHR in F, but it would probably be more expensive.
So by them putting F all the way, they get your money, you get a bit of a lower service AKL - MEL/SYD, but then you have a through transit experience in MEL/SYD, and probably have your bags tagged through direct, and the timing of the arr of the flight from AKL - MEL/SYD would be tied in as close as the dep for the flight from MEL/SYD to LHR.
I can see the benefit to QF doing it this way.
If by separate PNR, your bags would probably be able to be tagged as straight through, but your arr time in MEL/SYD might be hours apart from your dep from MEL/SYD to LHR via SIN, before "project sunrise" starts of course.