I don't think plat is used in Australia.
Looking in my trusty Shorter Oxford, I see several entries for the word, occupying about three quarters of a column, with meanings including a flat thing, a surface, a pleat or fold, a plait. However, most of these are identified as "Now arch. & dial.", "obs. exc. dial.", "obs. exc. Sc. dial.", etc.
The meaning "A plan or diagram of something; esp. a ground-plan of a building or of an area of land; a map, a chart" is dated from the early 16th Century, but labeled "Now US".