Beloved chain-smoking octogenarian monarch Margrethe II made a bombshell New Year's Eve announcement that she would be the first Danish royal to abdicate the throne in some 900 years.
Ahead of the royal handover on Sunday, there is a heavy police presence in the capital due to Denmark's high terrorism threat level.
Frederik and Mary met in Sydney during the 2000 Olympics. (AP PHOTO)
Tens of thousands of people are set to brave Copenhagen's 3C forecast temperature to catch a glimpse of the new monarchs.
Among the hundreds of early birds armed with red and white Danish flags are some blue ensigns with the Southern Cross; a nod to Mary's homeland.
Brisbane biomedical science graduate Ebony Wilson, 25, and her grandmother Judy bought a last-minute plane ticket to Copenhagen on Wednesday to be part of the historic moment.
''I was thinking about it and then I thought, why not be spontaneous? It's a once-in-a-lifetime opportunity,'' she told AAP in Copenhagen.
Brisbane graduate Ebony Wilson and her grandmother Judy travelled to Copenhagen for the succession. (Lisa Martin/AAP PHOTOS)
Royal enthusiast Rene Jensen 33, from Copenhagen, donned a plastic crown and a red cape over his winter jacket.
''I think (Mary and Frederik) are a powerful couple,'' he told AAP.
The Danish royal succession is set to be a low-key affair, with the main event about 1am (AEDT) when most Australians will be sleeping.
Frederik becomes Denmark's head of state, Mary will be Queen and their eldest son, 18-year-old Christian, the Crown Prince the moment Margrethe signs abdication papers at a state council meeting, marking the end of her 52-year reign.
King Frederik, 55, is scheduled to make his first public appearance on a balcony alongside the Danish prime minister who will formally proclaim him as the country's new regent.
The public hopes to also see Mary appear on the balcony.
Mary's Hobart-based sister Jane Stephens has travelled to Copenhagen.
Mary's elevation to Queen comes 23 years after she met the Danish Crown Prince in a Sydney pub during the 2000 Olympics.
Mary and Frederik married in May 2004 and have four children.
Crowds are gathering in Copenhagen's Christiansborg Castle square ahead of the Queen's abdication. (AP PHOTO)
The royal couple will ride home to their residence at Amalienborg Palace in a black-lacquered 1891 mahogany horse-drawn coach, while across the harbour military personnel will fire cannons in a three-by-27-shot honorary salute.
There will be more celebratory explosions later as Copenhagen's famed Tivoli Gardens amusement park hosts a fireworks extravaganza - the biggest in its 180-year history.
Aussie expats on the Australian Embassy's email database known affectionately as the ''Vegemite List'' are joining Ambassador Kerin Ayyalaraju for a soiree to toast Mary's promotion.
''I hope more Danes will be inspired to visit Australia to see where their new Queen was born and raised,'' Ms Ayyalaraju said.
The royal handover has generated a major tourism boost to the capital with a surge in hotel and rail bookings, according to tourism body Wonderful Copenhagen.
In Australia, events are planned at Sydney's Slip Inn where the two first met, and in Mary's home state of Tasmania, where landmarks will be lit in Denmark's red and white colours.
Opposition Leader Peter Dutton said Australians were proud the nation would have its own queen.
"Princess Mary is somebody who has been for a long period of time capturing the hearts of people around the world," he said.
