Search for one way flights:not a return
Your options are limited.
Consider separate ticket BOS to LAX or USA-Aust and a separate trans Tasman flight

Airlines that fly USA/Canada to Australia (most from LAX) are
- United [Star Alliance] (LAX SFO – SYD MEL & GUM-CNS)
- Qantas [OneWorld Alliance] (AS partner) (LAX JFK DFW HNL – SYD BNE MEL)
- Jetstar (HNL – SYD BNE)
- Delta [Skyteam Alliance] (VA & AS partner) (LAX – SYD)
- Virgin Australia (DL partner) (LAX – SYD BNE)
- Hawaiian, via Hawaii (AA & VA partner) (HNL – SYD)
- Air NZ, via AKL [Star Alliance]
- Fiji Airways, via Fiji (AA & AS partner)
- Air Tahiti Nui (last segments on codeshares) (AA & DL partner)
- Air Canada [Star Alliance]
And others via Asia & Middle East

Airlines that fly USA/Canada to New Zealand are
- Qantas via SYD BNE & MEL [OneWorld Alliance] (AS partner)
- Virgin Australia via BNE MEL & SYD (DL partner)
- Hawaiian (AA & VA partner)
- Air NZ [Star Alliance]
- Fiji Airways, via Fiji (AA & AS partner)
- Air Tahiti Nui via Tahiti (AKL) (AA & DL partner)
And others via Asia & Middle East