Australia's Princess Mary about to become queen of Denmark

Princess Mary has started her short and relatively low-key journey to join her husband, Prince Frederik, on the Danish throne and become the first Australian-born queen.

On a cold but clear Copenhagen day, Mary Donaldson is about to become the queen of Denmark while her native Tasmania sleeps.

The odd Australian flag fluttered amid crowds of excited Danes along Toldbodgade as the royal couple set out from Frederik VIII's Palace, Amalienborg, to Christiansborg Palace just after 11.35pm (11.35pm AEDT) on Sunday.

Minutes behind, Queen Margrethe, who set this historic day in action with a New Year's Eve speech announcing her intention to abdicate after 52 years on the throne, started her own ride to end her reign.

The outgoing royal beamed and waved a gloved hand from inside a grand black carriage accompanied by tens of thousands of wellwishers on her short journey to end her reign

The horseback Guard Hussar Regiment and police on motorbikes accompanied the tassled coach on the queen's kilometre-long journey from the neighbouring Christian IX's Palace to Christiansborg.

From there, Margrethe was set to sign a declaration of her abdication during a Council of State meeting at Christiansborg Palace.

Mary was expected to be present with her husband for the moment, along with their oldest son, Prince Christian, and members of the government.

After a small function for specially invited guests, the new king and queen will be proclaimed from the balcony of Christiansborg Palace at 3pm (1am AEDT), followed by a short speech and a cannon salute.

Danish Prime Minister Mette Frederiksen will step onto the balcony to proclaim the new monarchy.

Later, there will be a formal transfer of the royal colours and a reception to celebrate the accession.

Mary famously met the prince who will become King Frederik X, in a Sydney pub 24 years ago.

Rehearsals at Christiansborg Palace took place ahead of the understated event.

They included not only the royal horse guards but a stand-in carriage for the one that will carry one monarch in and a new king and queen out.

Copenhagen's streets feature a few hastily improvised nods to the royal shakeup.

One that has locals queuing up is at La Glace bakery, which has a 150-year history of making cakes in tribute to the royals.

After selling Queen Margrethe cakes for 52 years, they've had days to come up with a recipe for King Frederick.

Opinion polls show when it comes to who's the most popular royal after Margrethe, it's Mary.

Denmark's royal transition was sparked just weeks ago by her bombshell announcement on New Year's Eve, revealing her intention to abdicate within weeks.

The news that Frederik's hugely popular mother, the world's only reigning queen, would relinquish the throne, shocked Danes.

She will step aside 52 years to the day after she ascended the throne.

Only the main royal family and the Danish prime minister are likely to be in attendance.

The handover of power could hardly be more simple.

Margrethe will sign a document of abdication, and at that moment Frederick becomes King Frederick X and Mary becomes Queen.

Mary was born in Hobart, the youngest of four children of two Scottish academics.

Mary's father, John, is a professor of applied mathematics while her late mother Henrietta worked as the executive assistant to the vice-chancellor of The University of Tasmania.

Mary worked as an estate agent before meeting Frederik.