Because the same questions are asked over and over again and no one reviews the specific pinned topics or previous posts on the same subject. ;) and then there are 50 comments with one guy saying he didn't need insurance and another guy saying he did. No wonder the comments get closed pretty quick ;)
Also, You do not, and cannot, use a 30 day residence cert for any proof to give to the ATO (because virtually anyone can get one). You just need to prove, in Australia, that you qualify as a NON resident of Australia. Which is a whole different thing. A registered overseas address is good to put on your next tax return, but a good Accountant is much more important.