Green’s Dictionary of Slang

(Aus., Brisbane) a police car with a loud-speaker.

[1936		Telegram (Brisbane) 30 Apr. 18/7: Varsity procession in Hilarious Mood [...] The Carol Singers in their Police car, complete with loud-speaker and wireless equipment, gave all sorts of advice to pedestrians].
1941		Baker Popular Dict. Aus. Sl.
1955		N. Pulliam I Travelled a Lonely Land (1957) 231/2: carol singer – a police car equipped with a loud speaker.