Sorry about the delayed response, had a busy couple of days - you can do that if you keep it as pick up only, but in that case you won't be able to offer the item internationally.

A few years back, you could post an add on eBay without stipulating postage costs at all, even within Australia, however eBay put a stop to that as it did cause a few problems. Now, if you offer postage options within Australia, you must select the service and cost (the only way not to is to select 'freight' however even with that, the onus is on the seller to collect the cost of freight and arrange the shipment for the buyer). 

If you provide shipping options within Australia, you can then make it available to overseas buyers without specifying a shipping cost or method for them, but you can't do it for both within Aus and overseas (which you may have already figured out in the last couple of days  ).