Ok.... This is not an easy thing to find in black and white, and in clear simple terms. I read through the entire document that was referred to above (The Australian Consumer Law guide for consumers and lawyers). The problem with that document is, although it states all the rights consumers have and that they apply to online trading, the document deals solely with what rights consumers have after goods have been received by the consumer - there is nothing in the entire document that refers to what rights a consumer has if an item is purchased online and subsequently not received. 

There is obviously information about the obligations and responsibilities sellers have, but again they did not deal with lost mail items etc, the closest it got was whether a consumer has the right to claim for consequential loss if consumer guarantees are not met, which is a whole different kettle of fish. 

After all my searching and reading, the only thing I found was this:

Who is responsible for undelivered goods or damage in transit?

Read the delivery terms and conditions before you buy from an online seller. That information usually explains how such issues are handled and who is responsible if goods are not delivered or get damaged in transit. If you are not sure whether insurance is included in the cost of the goods or the shipping charges, email the seller about this before buying the goods.

While completing a sale, you might sometimes be given a choice of delivery options and even asked if you want to insure your goods at extra cost.

Contact the seller immediately if your goods have not arrived within the stated time or they are damaged.

That is a direct copy/paste from the NSW Fair Trading website (link: http://www.fairtrading.nsw.gov.au/Consumers/Ways_to_shop/Online_shopping.html), and while again it is not unequivocal proof of when delivery is deemed to have occurred, the question is phrased so that if the seller were legally responsible until the buyer physically receives the goods, the answer would be just that, and not "read the seller's policies".