Going out of Aust, not a problem 99.9% of the time.
Your bags can be put straight onto the CHC flight, ask in PER, so no farfing around with the bags on the QF transfer bus.
I have arrived into SYD D and rushed across to SYD I on the QF bus to CHC before, but my ADL - SYD flight arrived about 16.10/16.15.
I am always HLO. If you are too, good, if you have plane hold bags, with the transfer bus, your plane hold bags go automatically to the int terminal.
QF transfer bus is very good, as it goes via the internal road, on the inside of the airport fence. No traffic lights to worry about. Great view too.
As a J pax, you would have access to the SYD int J lounge, do make a 15 mins pop in, if you have the time.
QF SYD - CHC uses a narrow plane, the last time I did it, it left from gate 25 or so, I think so not long a walk.
If you are coming back to SYD, I would try the EK CHC - SYD service. Have done it in J, the bar access was a great thing.
QF does not have a lounge of its own at CHC, so its the Manaia, can't complain about that.
Pay the old Anglican Cathedral at Cathedral Square in CHC a visit, only birds live in there now!
Things are getting better in the city, but lots of construction work still going on.
If not driving on your midnight arrival, Supershuttle is a good one to use to the city, or the Sudima Hotel for the first night.