Hi and welcome to the forums.
I'm sorry to tell you that you'll have to replace your seats with US carseats. There is no way to certify seats from other countries - certification only happens at the factory during the production process, once it's off the assembly line, that's it - whatever country it's certified for is the country it stays certified for.
There are actually some significant differences between US and Australian seats, the biggest of which would be that there are seats that rf as high as 45lbs. My understanding is that in Australia most seats only rear-face to 20lbs and it's typical to ff a child at 9months. So you'll have a whole new world of options available to you here.
Feel free to start a new thread if you'd like some advice on what seats to pick. Just be sure to post the kind of vehicle you drive and your kiddo's weights - that will help with making more specific suggestions. Since you'll be buying US seats, feel free to start your new thread in the main carseat forum. :thumbsup: