Green’s Dictionary of Slang

(N.Z.) a beer truck.

1973		C. Wheeler Historic Sheep Stations NI 102: At this stage I remember [...] the ‘flagon wagon’ arriving at Te Paki from Houhora, New Zealand’s northernmost pub [DNZE].
2003		McGill Reed Dict. of N.Z. Sl. 79: flagon party The partygoers bring half-gallon glass jars of draught beer known as flagon beer; the host might surprise them with a flagon wagon, a truck carrying crates of flagons, latterly a large keg or container of beer that can be siphoned direct to consumer. Latter C20.