Online shopping platforms eBay and Groupon have converged with their offline retail peers by opening pop-up stores to offer instant shopping to beat the Christmas rush and ensure delivery before the crucial date of December 24.

EBay has launched its first Australian interactive store on the forecourt of Customs House, Sydney for the next four days.

Groupon has taken a short term lease of the former golf range store in Elizabeth Street to offer customers the option to buy in-store and collect instantly, or buy online and pick up at a time convenient to them.

The store will offers up to 70 per cent off more than 20 products to "click and collect", with a view to increase the selection to 30 products in the lead-up to Christmas Eve.

Groupon Australia chief executive Alistair Venn said the opening a bricks and mortar asset works to give the Groupon brand a physical presence as well as operationally allow for a free "click and collect" service to customers, "a trend we've seen locally and globally".

In-house research by eBay forecast that Sunday will be the busiest shopping day for the year, with estimates six items will be purchased online every six seconds, based on previous year sales, forward orders and eBay's own "traffic trends".

Megan English, eBay spokesperson, said the Christmas windows are instantly shoppable on a mobile phone 24/7 for the four-day period.

"This is the future of shopping – bringing the world of online, offline, and creating a seamless shopping experience for people to browse items in the windows, then shop them directly from their mobile," Ms English said.

"Our expectations are that on Sunday we will have 2.6 million online visitors, of which about 1.5 million will be using a mobile phone, to make the purchase. For extra efficiency, eBay has now joined with key retailers Dick Smith, The Good Guys, Supercheap Auto, BCF, Cincotta Chemist, and Masters, with more to come, for instant in-store pickup in time for Christmas."

Tim MacKinnon, eBay merchant development director said the launch of eBay's In-Store Pick Up initiative in Australia has been a key priority for the group, not only because it enhances the experience for shoppers, but also because it helps the retail partners get more traffic into their stores and  leverage the advantages they have over overseas retailers.

"The Australian consumer is particularly savvy with unique behaviours that demand constant innovation. We know that the eBay shopper in Australia spends more as percentage via mobile than any other eBay market, meaning they are always shopping on the go. If they can purchase something on the train on the way to work and pick it up in their lunch break we are taking some of the friction out of the shopping process and creating more opportunities for our retail partners to best service their needs," he said.