You probably do need an IRS form. Reason being that a US payer must withhold 30% of the payment to you as income tax and remit it to the IRS. You're then expected to file a refund claim with the IRS to get it back, unless your own country will let you deduct it from your local tax liability (and even that only if your local tax liability on that money is at least 30%). You don't have to, of course, but then you're losing your money to the benefit of the US debt reduction.
In addition, Australia has a tax treaty with the US, which may limit or even eliminate that withholding altogether. In order to claim your treaty benefit you must submit form W8-BEN to your employer, and specify the treaty and the article of the treaty on lines 9 and 10.
I suggest you find a CPA who is US-licensed and works in Australia or with Australians to help you with this. Dealing with the treaty may be problematic if you don't know what you're doing. I'm pretty sure a professional adviser who deals with your type of clients will be able to help you quickly and inexpensively.
American taxes are more horrific than any horror movie you've ever seen. Welcome to our world. We deal with the IRS so often that some of us even consider it being the normal way of things...