Department stores like Myers or David Jones are expecting millions to take part in the Boxing Day sales, while the National Retail Association predicts Australian shoppers will spend $2.62 billion on December 26 alone.

Roy Morgan's annual Christmas retail sales forecast a total $52.7 billion spending over the season.

In Sydney on Thursday, the Australian Retailers Association's executive director Russell Zimmerman had been in the CBD from about 5am, when Myer opened its doors an hour ahead of David Jones.

He said there had been a reasonable crowd but it probably wasn't as big as he'd seen in previous years.

"I think the reason for that is though that we now know, and we've had it for a number of years now, that the whole of NSW is open for business," Mr Zimmerman told AAP.

"And because of that, I think that the crowds have now dispersed themselves through the regional shopping centres and don't have to come into the city."

He said there was a possibility that pre-Christmas sales - including the recent Black Friday sales - would impact the popularity of the Boxing Day event.

"But as I said earlier, people love the atmosphere of Boxing Day and that may then mean that people still come out and shop," he said.

"We also need to remember that at 6 o'clock on Christmas Eve, you could actually get online and start your Boxing Day sales."

Shoppers headed to Melbourne's CBD early to get first dibs on the sale items, prepared with prams, grocery trolleys and comfortable shoes.

Daniel Keca was among the many resting their feet and wallets after his 6am start.

"I got what I need. Just if it feels like a good sale, I'll get it. Just see what it's like," the seasoned Boxing Day shopper said.

But it was a line for Lotte Duty Free, which snaked down Swanston Street into Bourke Street Mall, that had hundreds of customers willing to wait.

Chloe Gong told AAP the "good deal" on skincare was too good not to line up.

Meanwhile, research commissioned by Gumtree has found 54 per cent of Australian adults will receive at least one unwanted Christmas gift - and they'll be looking to cash in.

The estimated value of these items averaged $87, with incorrectly sized clothes topping the list.

"With more than half of Australians receiving unwanted gifts, there are plenty of people cashing in items to buy something they really want," Gumtree Australia spokeswoman Amanda Behre said.

Boxing Day is also forecast to become the busiest day on the National Broadband Network system.

Last Boxing Day, NBN registered an average amount of data consumed per household of 11.8GB, which meant a 69 per cent increase on the daily average of 6.98GB per household during 2018.

Australian Associated Press