The crew on CommCourt is correct - you don't need to carry out special service, but you really should be seeking a location order as part of your application, anyway, if you don't know where the ex and your kid are living.
In the orders you're seeking - both interm and final - ask for something along the lines of 'That both parties shall forthwith inform the other in writing of their respective current residential address, mobile phone number and e-mail address, and shall advise of any change to residential address, mobile phone number and/or e-mail address within 24 hours of such change occurring.'
You can seek a Commonwealth information order, but the Court won't necessarily disclose the information gathered from such an order to you, simply because it won't yet know whether it's in the child's best interests for such disclosure to be made. Instead, it will force government authorities, such as Centrelink, DHS or Medicare to disclose the last known address of the other party to the Family Court Registrar so that the other party can be served with Court documents.
Get legal advice on this. Legal Aid offers free consultations on family law matters, so it's worthwhile organising an appointment with them to discuss.