Australia uses 230V single phase power.
Either your X10 powerline interface is putting out a weak signal, or more likely, there is something else in your house that is generating noise or filtering out the signal. It could be anything from LED bulbs, a UPS, TV set or any device with electronics.
One way to narrow it down is to shut off everything at the breaker panel except for the 2 circuits that the X10 signal has to work across, and see if that works. If it does, then start turning other circuits back on one at a time and see when it stops working. Then, start disconnecting individual devices until you find what's causing the problem.