is it illegal to honk for no reason

Short answer: yes—blasting the horn for no reason is illegal in Australia. The road rules only allow honking to warn other road users or animals, or when a horn is part of a vehicle alarm/anti-theft system. Tooting to say hi, vent at traffic, or conduct an impromptu concerto is technically an offence and usually earns a small fine (amount varies by state). No demerit points in most places, but why donate to state revenue? If there’s actual danger, honk away; otherwise, let the horn have a quiet life.