If you’re an Australian business with mostly Australian customers then .com.au is best. It’s the most used and accepted extension for businesses in Australia. Also it helps with geo targeting in local search engine results. It’s also a good idea to grab the .com if it’s available.

If you’re doing business internationally or in the US, then .com is best. But if it’s not available, it can cost 10 times as much as .com.au to purchase off of someone.

The .net.au extension is for Australian businesses that can’t get the .com.au. They are easy to find available or acquire. They will do the job but you run the risk of customers going to the .com.au version of your website.