Green’s Dictionary of Slang

(Aus.) a multi-doored truck in which nightsoil pans were collected.

1970		J.S. Gunn ‘Twentieth-century Australian idiom’ in Ramson ed. English Transported 50: [T]here is no difficulty in adding to her list with terms like [...] fly swisher stew, ‘oxtail soup,’ honey cart or 17-door sedan, ‘sanitary cart,’ face plaster, ‘alcoholic drink,’ and dozens of others.
1978		G.A. Wilkes Dict. Aust. Colloq (1985)) 430/2: twenty-four door sedan A nightcart. 1978 Encountered in conversation.
1981		Macquarie Dict. 1578/1: seventeen door sedan [...] n. Colloq. > sanitary cart.
2004		W. Aubrey interview in Australians at War Film Archive 16 Apr. They didn’t even collect the sewer in those days, in the early days. It came later. Not the sewer itself but the pan man, what they called the ‘24-door sedan’. He used to come and collect the pa.
2011		J. Gibson Greensborough Historical Society: Oral History n.p.: Most homes had pan lavatories in an outdoor loo in the back yard. The nightsoilman would transport filled pans by balancing them on his head. He drove what was called a ‘24 door sedan’ which was a truck fitted with ranks of cubicles for the pan.
2016		P. O’Neill Facebook: Lost Brisbane 🌐 I grew up in NSW and Dunny Carts were also known as 24 Door Sedans as the cans were stored in separate compartments some of them even had pin striping around each door, they looked quite smart.