The principal need for two or more domains is to protect your business.
Just suppose for a moment that you have a great new business plan You setup a website name mygreatplan.com.au and it immediately shows a handsome profit. You have a right to be pleased with yourself. But, then business starts to taper off and you wonder why, until a customer suggests you should charge what your website is showing. When you show him your website and you are charging the correct price, he says, "No not that one , the other one, mygreatplan.net.au".
Someone has stolen your great plan and setup a website, mygreatplan.net.au. What can you do. maybe go to a good solicitor and get some advice, and any court action is also going to cost. Otherwise you have to live with it or develop a better, newer, greater plan.
So, what should you have done. Purchase the domains you need to protect your great plan, mygreatplan.com.au together with mygreatplan.net.au and the new domain mygreatplan.au. Total cost would be $15 AUD each, and you are protected. Much cheaper that court fees and lost profit.