The interval is probably assymetric for the same reasons that the UK has 230 +10/-6% and Sweden has +6/-10%. As New Zealand was already on 230V, it would have been rather odd of Australia to ask NZ to switch to 240V to create a common market...

You can check the year the standard was issued.

Somewhere on the net there is a pdf with results from test conducted prior to the change. I didn't bookmark it. [Linked Image] The Australians tested (new) 240V equipment (computers etc.) to see what it really worked with. The result was that it worked better with undervoltage than with overvoltage. This hinted that the 240V marking was a marking, not the design voltage.

Dapo might know more about this.