EK SYD - CHC dep at 8.50am, your MEL - SYD assume on time arr at 6.50am, ie, dep MEL at 6am, if for what ever reason, the MEL dep is late, this will "eat" into the connection time.
Edit: just had a check, its a 5.30am dep from MEL, but gets to SYD at 6.55am, so worst, best to over night, not so bad if the MEL - SYD flight was due to arr at 6.30am.
If you can, I would, if I were you, and done it several times, even if have to be on different PNR, do a QFd MEL - SYD the night before, stay at the Rydges ($$$ I know, have done it), and then go across to SYD T1 at 6am to enjoy the EK and QF J lounges.
As no linked PNR for BP from SYD T3, to get to T1, you can get the Tbus, have done that too.
Coming back, the EK CHC - SYD flight usually arr at about 6.45pm/7pm, so still enough time to get the last flight out of SYD - MEL, but I have done the overnight at the Ibis budget Sydney Airport on the way back several times, also into the city hotels several times.
This was all pre covid, havent been to CHC post covid.
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Aud$1119
MEL - SYD QF474 (31MAY2024) 17:30 18:55
SYD - CHC QF8766 (01JUN2024) 07:50 12:55
Linked PNR so you can get the QF transfer bus from the internal secure zone, at SYD T3 to get to T1 faster, as its most likely you might have the next flight BP, (unless they pull the "we need to obtain more information from you), which they might, but if you do get the SYD - CHC BP, the internal QF bus is the fastest.
$1119 is not that bad, and you get to use the QFd J lounge in MEL too.
If HLO only that is.
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The diff in that hr, is probably due to daylight savings.