I'm not sure a trumpet is allowed on as carryon in Australia.
As mad as it is, most sporting goods are currently banned. Including "sporting goods that could be used to harm including bats, sticks, poles, rods and spikes". Some musical instruments could pose similar threats to airline security.
Best you chat to AirNZ and ask their opinion. While it may not be easy to threaten someone with a trumpet, they still might view it as a credible (cough) threat.