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Just moved to OZ and converted my FAA CPL. I had to take a written CPL test - Air Rules and Flight Law only (there are a handful of different written tests pilots have to take here) and a flight test. Due to cost (all cost figures in this message are in Aussie dollars. Multiply .75 to get US), I did my test in an R22 ($390 per hour) and had to get current in the heli again. It had been two years since I had flown one and I was amazed how different it felt.
I did a couple of cross countries with my Aussie instructor and maneauvers until he was satisfied I could pass the test. In total, I think it cost me $5,000 to convert the CPL. I couldn't believe it, but aviation is more expensive here.
Also, you'll need to get a Night VFR rating here. For some reason, it's not included in your CPL training. Even with over 100 night hours and the training I received (and gave as a CFI) in the states, I had to take a flight test for the night VFR rating. I got off cheap borrowing a friends R44 at $580 per hour (schools charge $880 per hour)(finding an R22 configured for night flying was very difficult). Total cost ended up being $1400. Once again, I couldn't believe it.
I have an FAA instrument rating still to convert. I have an FAA CFII in helicopters, an FAA instrument airplane rating and around 55 simulated instrument hours. The requirements to convert are an instrument written and a flight test. Of course, I'm not current so I'll have to sit in the sim for a while before my flight test. The kicker though is CASA (FAA in OZ) requires IFR training be completed in an IFR approved machine. That eliminates R22s and R44s. The cheapest IFR approved aircraft I can find is a Jetranger at $1500 per hour. Ouch!! The flight test will probably be a couple of hours so I'm looking at $3500 to $4500 total to convert the rating. I can't believe it. How much it would cost someone getting their initial instrument rating here in OZ? It's no wonder most civil heli pilots do not have their instrument rating here. Side note: this is only for a single engine instrument rating. The Aussies classify multi-engine instrument as a different rating!
Don't even get me started on the job prospects! OZ has not experienced the pilot shortage the US has started to feel. Multi-engine job without multi-engine experience? Forget about it or stand in line for six years or more!
My suggestion is to remain in the US until you have all of the experience you need to get the job you want in Australia. The good news is the pilot shortage is headed to OZ in the future. When it hits, those who already have the experience will be in a really good position. Cheers!