To list an item on another site, you just have to log in to that particular site - your listings will be in the respective country's currency (so you need to make sure your PayPal account is set up to accept that currency), and subject to a few different policies, so it's a good idea to read up on any relevant policies, including eBay Buyer Protection if listing on a site that offers it to buyers.
If you would just like to make sure your currrent Australian listings have a chance of appearing in standard search results on the US and UK sites, you need to specify a postage service and cost to those countries separately from a worldwide flat rate.
