If it doesn't affect anything that's important to you, I would suggest increasing your handling time, even if you mainain the 1-business day posting. That will adjust the ETA on listings by the extra time, and likely provide a more accurate delivery time frame to buyers - you can adjust handling time in bulk, so just as easy as having a message in listings, but has the added benefit of (supposedly) preventing buyers from opening INR cases too early (the request can be opened one day after eBay's latest ETA).
My response to INR messages tends to depend on how long it's been since posting, but if it's still early days, rather than just ask them to wait longer, I say something along the lines of "I'd like to give it until XX date, just to allow for any Aus Post processing delays, and if it still hasn't arrived by then, please let me know and I will (all the usual options that I offer)".
That way, they get some assurance that something will be done if the item is more than likely not going to turn up, and by a set date, which helps put them at ease and therefore tends to make waiting a bit longer a little more acceptable.
