Mr. O’Leary pointed out that at Amazon’s debut, the Australian marketplace had only a few hundred sellers — or third-party vendors — who are now listing their products on the site. Globally, the company has millions of sellers, a volume seen as key to Amazon’s formula of providing large assortments of products at ruthlessly competitive prices.

As the number of sellers increases — and the company’s pricing algorithm takes hold — its penetration into the marketplace will improve significantly, he said.

Major Australian retailers, in the meantime, have long since begun preparing for Amazon’s entry into the marketplace. Some, like the electronics retailer JB Hi-Fi, expanded shipping offerings to include same-day delivery in certain regions.

Others, like Harvey Norman, have doubled down on their physical presences — the company has placed what it calls flagship stores in Ireland and throughout the Asia-Pacific region. Gerry Harvey, the company’s chairman, had previously said that Amazon would be “idiots” to begin its retail offering so close to Christmas.

“We haven’t got to adjust to anything,” said Mr. Harvey, who insisted that his business had not altered its plans in anticipation of Amazon’s arrival. “We’ve been in business now for 56 years. I’ve always been keen and able to be ahead of the pack. The minute someone gets ahead of us, I’ll hand the badge in. They follow us, we don’t follow them.”

Mr. Harvey complained about the “free publicity” that Amazon had received in the lead-up to the debut, and said their retail offerings had never posted a profit. (The company’s strategy is to build market share and pump the cash generated by its business into new growth initiatives like video streaming and devices.)