Green’s Dictionary of Slang

bun wagon n.

[? SE bun; or ? buns n. (1)]

(Aus.) a police van, a Black Maria n. (1)

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[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 21: Bun Waggon Police trawler.
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Bun wagon. Police or prison van used for transporting prisoners.

In phrases

capsize the bun-waggon (v.)

(Aus.) to create a surprise, to go against expectations.

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[Aus]Sydney Sportsman (Surry Hills, NSW) 7 Feb. 8/5: The next event, the 14.2 Handicap, resulted in a capsizing of the bun-waggon. Ophir, who was pretty well friendless, won nicely from the hot favorite, Miss Carbeau.