http://www.infrastructure.gov.au/roa..._1972_2018.pdf
Above is a link to a summary of emissions requirements for new petrol cars in Australia.
I've been searching the web and talking to my Engineering about the planned engine for my Pace 427 Build, a 351 EFI Windsor. The engine must pass an IM240 test and there is no magic way of passing you need a good tune, good cats and stock ECU. I have read about some modified turbo engines that have passed the IM240 so it seems to be all about what's coming at the tail pipe, emissions and noise!
Like others have said the engine is the easy part : )
Enjoy your journey