They may do it, similar to how EK use to do the SYD-AKL sector (DXB-SYD rest SYD-AKL-SYD rest SYD-DXB) EK now have an overnight rest in AKL...
So, in the UA example, it may be something like SFO/LAX-SYD; overnight in SYD. SYD-MEL-SYD overnight in SYD, then the following day fly out to LAX?SFO...
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