good, AC is a perfectly valid routing with MX.

On their system they'll come up with the choice of

UA, AC and NZ.

And yes, you may be able to do

MEL-SYD (on UA), SYD-HNL-YVR-JFK

Though, award availability is very very tough on the HNL-YVR sector, very tough! (in Y at least, they have better availability in C).

You could also try

SYD-HNL (on AC) - SFO or LAX (on UA) through to JFK or EWR.