Most items that use a power pack or power brick style of power supply can be used with any voltage, but not all plug packs are "universal" -- some are good only with one voltage. Check yours to be sure. With these power pack/brick style of power supply that can use 240 volts it is often just a case of getting a new power cord with an Australian plug.
These examples show the typical connectors for these types of power brick/pack style of power supplies C5 Style

Most computers also can handle both 110v and 220v and only require a new power cord with an Australian plug.
C13/C14
Plug Adapter
With items that can handle both 110v and 240v after you use a new power cord or a plug adapter these items should be safe to use in Australia.
Many people have old power cords around the house that can be re-tasked to your new purchase however you can also buy them at most computer stores, Harvey Norman, Dick Smiths or Tandy. Plug adapters can be purchased in even more locations and you can even buy it though Amazon.
For items that cannot handle the Australian 240v mains power as it has only been designed for the USA market. then transformers are available. You can purchase them here in Australia or through Amazon. The size of the transformer required will depend on the power draw of the item itself so insure you research exactly what your power requirements are before purchasing.
Transformers for low power usage items are quite small and are often used by international travellers. Transformers for items with much higher power requirements are quite large, heavy, expensive, and heat up noticeably.
This site shows you just what items on the Amazon store can be shipped to Australia.
When you have found what you want you will be transferred to Amazon.com to complete the purchase.
Amazon is the world's largest online store and you all purchasing, checkout and payment processing is done by them for your security
Shipping Costs to Australia with Amazon start from as little as $4.99