I had no trouble pricing BOS-SFO-LAX-MEL-BNE on ua.com. I used multi-city with a city break in SFO (i.e. BOS-SFO, SFO-BNE, BNE-SFO, SFO-BOS). United doesn't want to sell tickets from MEL-BNE as a "stand-alone" segment probably because either no *A option or no UA fare filed.
To get SFO-BNE to be SFO-LAX-MEL-BNE just uncheck SYD and AKL as connection points in the filter. You can also route BOS(-IAH)-SFO-AKL-BNE with the same search. Prices in D with a r/t cost of about $9.5k.
I would recommend searching ITA to make sure you have space too - first AKL attempt I tried forced J on a segment probably because no D space.