Christmas shoppers in the Sydney CBD.

Festive: Shoppers in the CBD. Photo: Dallas Kilponen

Retailers are hoping for the best Christmas in five years with this festive season's sales expected to increase by 5.4 per cent, compared with the corresponding period last year.

The Australian National Retailers Association says $20 billion has already rung through the cash registers this Christmas period but the most intense days of shopping are yet to come.

"Individually, we anticipate shoppers would have spent around $840 per person so far on Christmas preparations," said the association's chief, Margy Osmond, referring to the period between November 25 and December 15.

Merinda Rousso and Josh Renfrey.

'Tis the season: Merinda Rousso and Joshua Renfrey by the Christmas tree in Martin Place that Ms Rousso used for her marriage proposal. Photo: Fiona Morris

The largest rise in Christmas spending will be through internet shopping, with an expected 18 per cent sale increase on domestic sites and a 20 per cent increase on overseas sites compared with last year.

But not everyone is stuck in queues this week - many are out enjoying Christmas projects that cost the Sydney Council $2.1 million this year to guarantee the city becomes a destination for Christmas shoppers and their families.

Merinda Rousso planned to propose to her boyfriend on December 25 but all that changed when she was charmed by the world's first interactive Christmas tree.

In Martin Place last week she decided to seize the moment, using the tree's text messaging system to send her marriage proposal spiralling up the 20-metre pine in fluorescent light for her boyfriend, Joshua Renfrey, and all of the late-night shoppers to see. "It was a beautiful night and the tree was just beautiful so it was just the perfect moment," said Ms Rousso.

"I'm glad he said yes," said Tim Clarkson, from Chas Clarkson, who digitalised the Martin Place Christmas tree.

From one of the city's newest Christmas attractions to the oldest, the David Jones Christmas windows take a creative team a year to design and construct. Each December Kate Charles, the visual merchandising manager at David Jones, flies to inspect department store windows in Los Angeles, New York, London and Paris to draw inspiration and ensure that the DJs windows remain at world class standard.

To mark their 175th birthday this year, the David Jones windows feature French marionette puppets acting out important moments in the store's history.

''For example, we had a state dinner on the seventh floor in the Elizabeth Street store in 1954 so we have a puppet showing that moment of Queen Elizabeth II arriving into the dinner,'' Ms Charles said. Each year sound engineers compose music and sound effects to accompany the displays. "First we show them the windows and talk them through the story and then they write the music," she said. Music is integral to the city's Christmas branding this year.

Nimrod Weis, the creative director at Eness, a design company that creates interactive art installations, built a 16-character choir whose song amplifies depending on the size of the surrounding crowd in Pitt Street Mall.

"The City of Sydney Council wanted LED screens but we turned the screens into unique characters so we made it more accessible to the public,'' Mr Weis said.

"The composers had to create a new type of festive carol - that was the brief - then we matched the voices to the characters."

Correction: The original version of this story said Tim Clarkson was from The Electric Canvass.