I'm an Australian, and TBH if I was wondering whether to hook something up to the mains I would ask an electrician! Since you are one, I'm not sure where to go next, except to read whatever regulations you can get your hands on.
I know we have here (in my house) some relays which turn on things at a distance (like garden lights) and I assume that it would be safe enough / legal enough to fiddle with the low-voltage side (assuming it is 12 V or something like that), however that is pure guesswork on my part.
Since the Arduino can't directly switch or control 240 V, the issue for me would be: How legal/approved is the device you are planning to interpose between the Arduino and your mains supply?
I found this link, for example: http://acma.gov.au/Citizen/Consumer-info/My-connected-home/Fixed-line-phones/electrical-safety
If you are uncertain about operating a device in Australia, discuss your options with a suitably qualified electrician, licensed to operate in Australia.
Well, Tangles84, it looks like you will have to have a discussion with yourself. And if you don't know, you must know who you can ask, such as the place you got your training, or someone with a higher level license.
if I install all the wiring and certify the wiring and the relays that are rated for the fault current and has the isolation required is there any legal requirement for any certification for the controller.
My guess is that if you are using approved equipment, and that equipment is operated by a (say) 12 V signal, then they won't particularly care what provides that signal. However that is purely a guess. There might be issues if, say, the equipment controlled by the controller was turned on and off in a hazardous way (eg. very frequently) or in an unexpected way (eg. without warning).