The nominal voltage in most areas of Australia was set at 240 V in the 1920s. However, a change began in 1980 with the International Electrotechnical Commission (IEC) deciding to rationalise the 220 V, 230 V and 240 V nominal voltage levels around the world to a consistent 230 V.
This rationalisation was ostensibly made to improve the economics of making appliances by allowing manufacturers to produce a range of items with a rated voltage of 230 V. In 2000, Standards Australia issued a system Standard, AS60038, with 230V as the nominal voltage with a +10% to –6% variation at the point of supply. (253 V to 216.2 V)
A new power quality standard, AS61000.3.100, was released in 2011[22] that details requirements additional to the existing systems Standard. The new Standard stipulates a nominal 230 V, and the allowable voltage to the customer’s point of supply is, as mentioned, +10% to –6%. However, the preferred operating range is +6% to –2%. (243.8 V to 225.4 V) [23]
I suspect they are all made in the one factory by the one manufacturer - then the various brands just buy it and slap their brand label on it and box it.