It'll depend as to whether you have a post paid or a pre-paid service.
If you have a post-paid account, bills will be sent out (generally once a month) via email or traditional post and will serve as a tax invoice for the purposes of the ATO. If you need historical accounts, your previous 12 months of bills can be downloaded through My Account, here: https://tel.st/d8r9c. If you need further back than that, you'll need to call 13 22 00 and say "billing" at the prompt; and there may be a charges incurred to cover the cost of actioning the request.
If you have a pre-paid service it is slightly more difficult, as there's no charge until such time as your credit is used (in line with the Accounting principle that an expense isn't counted until it is incurred). You'll receive a receipt on purchasing credit, but whether the ATO will accept those is not information I can provide. Usage information for pre-paid services is also available through My Account.