As mentioned in other answers, you may not need to register your business in Australia just to deliver work to an Australian client. However, if you wish to, the steps are:
1) Register the business in order to get an Australian Business Number. The ABN is required for a variety of business-related operations such as opening bank accounts and issuing Tax Invoices. There is provision for ABNs to be issued to foreign companies who are doing business in Australia.
2) If you want to incorporate in Australia (rather than just have an ABN issued to your existing business), there are a variety of other requirements, which for your purposes would be purely administrative. You will need an Australian address - you may wish to use a virtual office supplier, such as Regus. For the process of incorporating, I used ClearDocs - a fully online business incorporation service, which cost around $600AUD. Most of that money goes towards registration fees required by the various government departments.
3) You may choose to register for GST with the Australian Tax Office (ATO). Companies which turn over more than $75,000 per year must register for GST, but even if you don't turn over that amount, you may choose to register. The benefit of GST registration is that you can charge GST to your customers and claim back GST credits on your business purchases.
4) Optionally, you may want to open an Australian bank account. This will reduce the administrative complexity for your customers to pay you, as they will be able to pay to an Australian account in Australian dollars.
5) Understanding the requirements for issuing Tax Invoices within Australia. This is the mechanism by which most companies interact financially and if you are freelancing, almost certainly the mechanism your customers will be expecting.
6) Engage an Australian accountant. There are approaches you can take to how you structure your business and how you collect and report your tax which you will need assistance with. You'll need to understand your Business Activity Statement (BAS) and other annual reporting requirements.